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PROFESSOR: -- All the things
would be relevant, so here

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are some examples from a
couple of lectures ago.

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And so here we're thinking
about what happens when

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we add an inert gas.
 

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And remember, it's all about
the partial pressure.

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So, you always have to ask
yourself did the partial

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pressure change, and partial
pressure's going to change if

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there's a change in volume.
 

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So, the secret in this problem
is realizing that if inert

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gas is added, and the total
pressure is kept constant,

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what had to have happened?
 

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Yup, the volume would have had
to increase, and so the system

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is put at stress, and it
responds in a way to

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minimize the stress.
 

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So it's going to respond in a
way to go from fewer numbers of

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molecules to more molecules.
 

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So, on one side of the equation
reactants there are three, and

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on products there are two, so
it's going to shift

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toward reactants.
 

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All right.
 

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So today we're going to have
another clicker competition,

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and because it is Halloween,
this is the prize for the

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recitation that has the
most correct answers.

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So, let's see how we do today.
 

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All right.
 

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So we're going to continue
where we left off on Wednesday,

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and so these are your notes --
I've added them to today's

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handout and they're also in the
handout from the last class.

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And I want to make a note that
it's a good idea to start

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this problem-set early.
 

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You don't know everything you
need to know to do the

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problem-set, but you do know a
number of them, so there's some

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questions on thermodynamics and
equilibrium, and Le Chatelier's

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principle, you can do
all those problems.

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So next week -- today we're
going to talk about bases and

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buffers, and then we're going
to move into acid-base

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titrations on Monday.
 

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And so, the problem-set looks
like it's not that long, it's

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not that many questions, but
the acid-base titrations have

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many parts, and each part
is actually quite long.

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So it's a very
deceptive problem-set.

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So don't be fooled by the
total number of questions.

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All right, so we were talking
about acid and water and base

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and water and p h, and so
we're going to continue with

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base and water right now.
 

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So here we have a base in
water, and so in this case, the

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water is acting as an acid.
 

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It's giving up a hydrogen ion
or proton to the n h 3, causing

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it to form its conjugate, n h 4
plus, ammonia ion, and

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also hydroxide ion.
 

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So here we have a base in
water, and when we're talking

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about a base in water, we're
going to talk about base

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ionization constant, or k b.
 

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So at the end of last class
we talked about the acid

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ionization constant, or k
a, and when you're talking

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about bases, you're going
to talk about k b's.

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So, k b, it's an equilibrium
constant for this reaction of a

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base in water, and so it'll be
equal to the products in h

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4 plus and hydroxide ion
over the reactant, n h 3.

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The water is the solvent here,
and since this is all pretty

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dilute, it's mostly pure and
its concentration isn't going

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to change, so it's not included
in that equilibrium constant.

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So we have a k b when we're
talking about a base in water.

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And we were talking about a
base in water, the equation

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should work that you have
hydroxide ions on

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one side of it.
 

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So here, the k b is 1 .
 

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8 times 10 to the minus 5
at 25 degrees, so that's

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a fairly small number.
 

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And so the small value tells us
that only a little bit of the n

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h 3 is going to ionize
when it's in solution.

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So only a little bit it's
going to form n h 4 plus

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and hydroxide ions.
 

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So that's what that
small number tells us.

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So that tells us that it's
going to be a weak base.

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So a strong base is something
that's going to react pretty

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much completely to go to
hydroxide ion concentration.

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A weak base only ionizes
a little bit in water.

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And you can tell about whether
something's strong or weak by

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its k b value, or if it's an
acid, it's k a value

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or p k a value.
 

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So here are some general ways
to write these equations.

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We have a base in water and so
the water's going to act as the

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acid, base is going to accept
that proton or hydrogen ion

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forming a base, h plus,
and hydroxide ion.

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So the base is just
written as b.

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You could also write the base
as a minus, something a minus

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in water going to h a
plus hydroxide ion.

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So sometimes you might see that
when you're talking about a

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conjugate base of a weak acid.
 

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So these are two expressions
that you'll see that are fairly

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generic that expresses what
happens when you have

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bases in water.
 

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Now remember, you know it's a
base in water, you better have

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hydroxide ions on the other
side, because a base in water

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is going to be forming
hydroxide ions, and acid in

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water would be forming
hydronium ions.

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So, a strong base, again,
almost completely ionizes to o

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h when in water, and here we
can know what's strong or weak

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by the k b, so the larger the
k b the stronger the base.

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And like there is the p k a,
there's also a term p k b.

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P k b is minus log of the k b.
 

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And the larger the p k
b, the weaker the base.

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Now you won't see
p k b very much.

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It's not used very
much, most things are

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converted to a p k a.
 

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So you'll see p k a's quite a
bit, and you will see p k a's

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if you take organic chemistry,
if you take biochemistry, if

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you take biology, you'll be
hearing a lot about p

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k a's as we go along.
 

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Not so much about p
k a's, but p k b's.

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And so what I want you to do
when you hear about p k a's,

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is remember that you've
learned about it.

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Because I have been confronted
by some of my colleagues who

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teach in advanced levels, and
they said our students tell us

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that you never talked about p
k a's in freshman chemistry.

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And I assure them that I did.
 

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So I'll be emphasizing this.
 

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And so I want to make sure that
by the end of this unit you're

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really familiar with p k a's
because you'll need them later

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on, and I want you to really
impress my colleagues in later

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classes, and they'll say oh,
that's a 511-1 student, of

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course, they know
what a p k a is.

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Even my six month old daughter
who's over there, she's

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like what, people didn't
know what a p k a is?

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You don't want to
get her upset.

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OK.
 

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So all of these things are
related with the acids and the

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bases, because for every acid
it has a conjugate base, every

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base is a conjugate acid.
 

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And so, if you have a stronger
acid, the stronger the acid,

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the weaker its conjugate base.
 

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And the stronger the base, the
weaker its conjugate acid.

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And this becomes very important
in doing these problems.

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So here's a little table
that emphasizes that fact.

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So we talk about a strong acid.
 

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Most people are familiar with
h c l, hydrochloric acid.

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So it's a very strong acid.
 

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And its conjugate base, c l
minus, is not really a base,

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it's completely ineffectual
as being a base.

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It doesn't really do
anything at all.

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A strong acid really drives
you all the way to hydronium

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ion concentrations.
 

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It doesn't go back the other
way, it's not really

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equilibrium, it's just
going to completion there.

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So the conjugate is really,
really weak, basically

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not a base at all.
 

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Then we get into this middle
range and here things that

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are moderately weak or very
weak acids also have their

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conjugates in the weak range.
 

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But if you get to something
that is a very strong base down

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here, its conjugate is going to
be also ineffective as an acid.

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So if something is very strong,
its conjugate is pretty much

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non-existent in those
properties, but when you have

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weak-weak, then you can start
talking about buffers, which

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we're going to get into
later in today's class.

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OK.
 

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So let's prove that, in fact,
it has to be true that there's

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a relationship between the
conjugate acid and its

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conjugate base or conjugate
base and its conjugate acid,

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that they both can't be strong.
 

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One has to be -- you
have to be weak-weak,

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strong or ineffectual.
 

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All right, so let's look
at the first one up here.

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So first let's look at
what is this acting as?

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What is n h 3 acting
as, an acid or a base

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in this equation?
 

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So it's acting as a base.
 

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And that means water
is acting as an acid.

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The water gives up a proton or
hydrogen ion to the n h 3

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forming the conjugate
acid of that base.

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And then the conjugate base
is the hydroxide ion.

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All right, so now let's write
term for k, and so we're

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talking about a base in water,
so we're talking about k b.

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So k b is going to equal what
-- what do I put up here?

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Tell me one thing
to put up there.

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Yup.
 

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OK, n h 4 plus, and
hydroxide ion over n h 3.

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OK, so we don't have
water in there.

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All right, so let's look
at the next reaction.

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So what is n h 4
plus acting as?

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It's acting as an acid here, so
it's giving up its proton or

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hydrogen ion to the water,
which is going to act as a base

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and accept that hydrogen atom.
 

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And when this gives up its
hydrogen ion or proton, it

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forms its conjugate base, and
the water is conjugate here,

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is an acid, hydronium ion.
 

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So we have our conjugate
acid base pairs here.

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So now am I talking
about a k a or a k b?

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I'm talking about a k a, so I'm
talking about an acid in water,

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and we know this is an acid in
water if we look at what's

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happening over here.
 

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So we have an acid in water,
and so we'll put our

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concentration of hydronium ions
and n h 3 over n h 4 plus.

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So now we have k a's and k b's
written for the conjugates, the

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conjugate acid of n h
3 and n h 3 itself.

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So now, we can think about
what happens if we take

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these k's and we multiply
them out together.

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All right, so we have a
k a and we have a k b.

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00:12:39 --> 00:12:43
All right, so we have a k a and
a k b, so if we take our k a

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00:12:43 --> 00:12:48
and times our k b, we're
going to just multiply

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these out together.
 

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So I'll do this one first, k a
-- just copy from above --

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00:13:04 --> 00:13:18
times k b, hydroxide
ion, over n h 3.

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And things are going to cancel
out, and so I'm left with

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00:13:28 --> 00:13:34
hydronium ion and
hydroxide ion.

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What is this, when you
have hydronium ion

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00:13:36 --> 00:13:37
times hydroxide ion?
 

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What is that called?
 

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00:13:39 --> 00:13:41
It's another k.
 

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00:13:41 --> 00:13:44
K w.
 

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So we just showed that k
a times k b equals k w.

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So we can take the logs of all
our k terms here, and if we

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00:13:58 --> 00:14:10
take the log of k a plus the
log of k b equal the log of

223
00:14:10 --> 00:14:33
k w, or p k a plus p k b
equals p k w equals 14.

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So there's this relationship
with a conjugate acid and its

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00:14:36 --> 00:14:39
base between its
k a and its k b.

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So if one is really big, the
other has to be small or

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they can both be sort
of in the middle.

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But they're always going to
add up in terms of the p

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00:14:48 --> 00:14:50
k a and the p k b to 14.
 

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And the thing about these
problems is if you're given a

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00:14:54 --> 00:14:59
k a for an acid, you can
calculate the k b for its

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conjugate base, and you'll be
doing that a lot in titration

233
00:15:02 --> 00:15:05
problems that are coming up.
 

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All right, so there's this
relationship between the

235
00:15:08 --> 00:15:11
strength of an acid and the
strength of its conjugate base,

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00:15:11 --> 00:15:15
and let's just think for a
minute again about this concept

237
00:15:15 --> 00:15:17
of strong and weak, because
this is really important

238
00:15:17 --> 00:15:19
for the next unit.
 

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00:15:19 --> 00:15:34
So if we have a strong acid, h
a, in water, it's going to go

240
00:15:34 --> 00:15:39
pretty much completely over to
hydronium ion and the

241
00:15:39 --> 00:15:44
conjugate, and this conjugate
is going to be really

242
00:15:44 --> 00:15:49
ineffective as a conjugate
base, as a base at all.

243
00:15:49 --> 00:15:52
So it's going to really
go all to that hydronium

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ion concentration.
 

245
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And so, in talking about a
strong base, you don't really

246
00:15:56 --> 00:15:59
have to worry about an
equilibrium situation.

247
00:15:59 --> 00:16:02
Just remember it goes pretty
much to completion, and so you

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00:16:02 --> 00:16:06
can do complete subtractions
when you're doing this.

249
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And the same is true
for a strong base.

250
00:16:09 --> 00:16:14
So, for a strong base, any b,
in water, it's going all the

251
00:16:14 --> 00:16:22
way down, it's driving the
reaction all the way over here,

252
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and you're forming, you can
consider it that however much

253
00:16:24 --> 00:16:34
strong base you added is how
much hydroxide ion

254
00:16:34 --> 00:16:35
you have here.
 

255
00:16:35 --> 00:16:39
How much strong acid you add is
equal to how much hydronium ion

256
00:16:39 --> 00:16:41
concentration you have here.
 

257
00:16:41 --> 00:16:45
So however much of a strong
acid or a strong base, you

258
00:16:45 --> 00:16:48
think they go all the way to
completion, but for a weak acid

259
00:16:48 --> 00:16:51
we're going to have
equilibrium, and so you'll have

260
00:16:51 --> 00:16:54
to set up equilibrium tables to
figure out if you added this

261
00:16:54 --> 00:16:58
much weak base, how
much did it ionize.

262
00:16:58 --> 00:17:02
So remember that -- people
get worried about the strong

263
00:17:02 --> 00:17:05
acid, and you just assume it
goes right to completion.

264
00:17:05 --> 00:17:10
And you can tell again by the
k a's and the k b's what's

265
00:17:10 --> 00:17:11
going to be strong or not.
 

266
00:17:11 --> 00:17:16
And so our definition for
strong acids is that you have a

267
00:17:16 --> 00:17:20
k a greater than one, strong
base pretty much, the only

268
00:17:20 --> 00:17:24
problems you use, people are
adding sodium hydroxide or

269
00:17:24 --> 00:17:27
potassium hydroxide, there are
not a lot of options

270
00:17:27 --> 00:17:28
for strong bases.
 

271
00:17:28 --> 00:17:31
But for strong acids, people
are always worrying about

272
00:17:31 --> 00:17:36
whether they've identified
those correctly or not.

273
00:17:36 --> 00:17:36
OK.
 

274
00:17:36 --> 00:17:42
So let's look at this relative
strength of acid problem

275
00:17:42 --> 00:17:46
and do an example here.
 

276
00:17:46 --> 00:17:52
So in this equation we have
acid in a base going on one

277
00:17:52 --> 00:17:55
side, another different
acid on the other side.

278
00:17:55 --> 00:18:00
So we can look at whether the
reaction is favored toward this

279
00:18:00 --> 00:18:03
direction, toward the right
or the left, depending on

280
00:18:03 --> 00:18:05
which acid is stronger.
 

281
00:18:05 --> 00:18:07
Will the reaction ride
to the right or left.

282
00:18:07 --> 00:18:11
So if this acid is stronger,
then it should drive

283
00:18:11 --> 00:18:12
the reaction this way.
 

284
00:18:12 --> 00:18:16
If this is the acid on the
other side, if it's stronger,

285
00:18:16 --> 00:18:19
then you would expect to drive
the reaction the other way.

286
00:18:19 --> 00:18:23
So we can take a look at that.
 

287
00:18:23 --> 00:18:27
So we can consider the k for
the overall reaction, again,

288
00:18:27 --> 00:18:30
just products over
reactants here.

289
00:18:30 --> 00:18:34
And we can also consider the
reaction from just each

290
00:18:34 --> 00:18:38
acid alone in water.
 

291
00:18:38 --> 00:18:41
So we can consider it
separately as well.

292
00:18:41 --> 00:18:45
So first we can take
a look at one acid.

293
00:18:45 --> 00:18:49
So if we take a look at this
acid alone in water, it'll form

294
00:18:49 --> 00:18:53
h 3 o plus and n h 3 minus.
 

295
00:18:53 --> 00:18:57
So when it gives up its
hydrogen ion to the water

296
00:18:57 --> 00:19:00
and then it forms its
conjugate over here.

297
00:19:00 --> 00:19:07
So now are we going to
look at a k a or a k b?

298
00:19:07 --> 00:19:07
K a?
 

299
00:19:07 --> 00:19:11
So we're going to have our
products over our reactants,

300
00:19:11 --> 00:19:16
and the number is quite
large, 20, for that k a.

301
00:19:16 --> 00:19:21
And now we can look at the
other reaction as well.

302
00:19:21 --> 00:19:25
So we can look at this acid in
water here and form your

303
00:19:25 --> 00:19:29
hydronium ion concentrations
and your conjugates -- again,

304
00:19:29 --> 00:19:33
we're looking at that acid in
water, so it's a k a, and we

305
00:19:33 --> 00:19:35
have our products
over reactants.

306
00:19:35 --> 00:19:37
Now we have a number of 5 .
 

307
00:19:37 --> 00:19:42
6 times 10 to the minus 10.
 

308
00:19:42 --> 00:19:48
So, we can consider those two
equations back together, and

309
00:19:48 --> 00:19:51
this time we're going to be
subtracting the equations from

310
00:19:51 --> 00:19:54
each other to get
our sum equation.

311
00:19:54 --> 00:19:56
And because we are subtracting,
we're going to end

312
00:19:56 --> 00:19:59
up to dividing the
equilibrium constants.

313
00:19:59 --> 00:20:03
So when we add the equilibrium
constants together, we multiply

314
00:20:03 --> 00:20:05
things, and if we're
subtracting we divide.

315
00:20:05 --> 00:20:10
So the k in this case is going
to be equal to the k a of the

316
00:20:10 --> 00:20:13
first acid over the second
acid, and you can prove this to

317
00:20:13 --> 00:20:18
yourself, you write up the k a
here, and the k a here, and

318
00:20:18 --> 00:20:22
then some of your terms, the
hydronium ions cancel, and you

319
00:20:22 --> 00:20:25
get the k overall equilibrium
constant that we wrote in the

320
00:20:25 --> 00:20:28
beginning, again products
over reactants.

321
00:20:28 --> 00:20:31
We know the value for k a for
one acid, we know the value of

322
00:20:31 --> 00:20:35
k a for the other acid, and we
can divide those to get the

323
00:20:35 --> 00:20:37
k for the overall reaction.
 

324
00:20:37 --> 00:20:42
And then you can tell me what
that k means in terms of which

325
00:20:42 --> 00:20:47
is the stronger acid of the
two, and which side of the

326
00:20:47 --> 00:20:51
equation does the reaction lie
to, the right or the left?

327
00:20:51 --> 00:21:18
All right, let's give
10 more seconds.

328
00:21:18 --> 00:21:35
Yup, people did
pretty well on this.

329
00:21:35 --> 00:21:38
So you could have thought about
it in terms of the overall k

330
00:21:38 --> 00:21:41
or of the individual k a's.
 

331
00:21:41 --> 00:21:45
So, the stronger acid is the
one with the larger number.

332
00:21:45 --> 00:21:49
And h n o 3 had a number of 20,
so that was pretty big, so

333
00:21:49 --> 00:21:52
that's a really strong acid.
 

334
00:21:52 --> 00:21:56
And because it's a strong acid,
it'll lie to the right, so it's

335
00:21:56 --> 00:21:59
going to push toward products
-- it's a strong acid, so it

336
00:21:59 --> 00:22:04
wants to disassociate a lot, so
would push it that direction.

337
00:22:04 --> 00:22:07
And you can see that also in
terms of the equilibrium.

338
00:22:07 --> 00:22:11
Overall equilibrium constant,
if we go back to the

339
00:22:11 --> 00:22:12
slides for a minute.
 

340
00:22:12 --> 00:22:17
So this number overall, k, is
also quite large, very large,

341
00:22:17 --> 00:22:21
so that means a lot more
products than reactants

342
00:22:21 --> 00:22:23
at equilibrium.
 

343
00:22:23 --> 00:22:27
So again, those are what you
can determine if you're given a

344
00:22:27 --> 00:22:32
table of k a values, which on
the test you will get a table

345
00:22:32 --> 00:22:36
of k a values, you can tell me
a lot about different reactions

346
00:22:36 --> 00:22:39
knowing that information
about k a's.

347
00:22:39 --> 00:22:43
All right.
 

348
00:22:43 --> 00:22:50
So in this unit there are
different types of acid base

349
00:22:50 --> 00:22:54
problems, and sometimes it
feels for people like there's

350
00:22:54 --> 00:22:57
an infinite number of different
types of acid base problems.

351
00:22:57 --> 00:23:00
But, in fact, there
are really only five.

352
00:23:00 --> 00:23:03
And so one of the things I
strongly recommend in this

353
00:23:03 --> 00:23:07
unit and working problems is
figuring out which type of

354
00:23:07 --> 00:23:12
problem it is, and that will
help you a lot in solving it.

355
00:23:12 --> 00:23:15
So you can either have a weak
acid in water, a weak base in

356
00:23:15 --> 00:23:19
water, and sometimes you can be
fooled and say oh, it's a salt

357
00:23:19 --> 00:23:22
and water problem, but a salt
and water problem actually

358
00:23:22 --> 00:23:25
breaks down to a weak acid and
water problem and a weak

359
00:23:25 --> 00:23:26
base and water problem.
 

360
00:23:26 --> 00:23:29
So it's really not a different
kind of problem, and we'll

361
00:23:29 --> 00:23:30
see that in a few minutes.
 

362
00:23:30 --> 00:23:33
And you can have a strong acid
in water and a strong base in

363
00:23:33 --> 00:23:37
water, and then you can have my
good friend the buffer

364
00:23:37 --> 00:23:38
type of problem.
 

365
00:23:38 --> 00:23:40
So those are the type of
problems and being able to

366
00:23:40 --> 00:23:44
recognize them is key to
doing well in this unit.

367
00:23:44 --> 00:23:47
So let's work a problem in
the first type of a weak

368
00:23:47 --> 00:23:50
acid in water problem.
 

369
00:23:50 --> 00:23:52
All right, so what's
a weak acid?

370
00:23:52 --> 00:23:55
Well, vitamin C is a weak acid.
 

371
00:23:55 --> 00:23:58
And so sometimes when you're
taking your vitamins you get

372
00:23:58 --> 00:24:00
a bad taste in your mouth.
 

373
00:24:00 --> 00:24:07
And if you did take a vitamin C
tablet here, which is 500

374
00:24:07 --> 00:24:10
milligrams in this vitamin C
tablet, and dissolved it in

375
00:24:10 --> 00:24:13
water -- this is not
scientifically measured -- but

376
00:24:13 --> 00:24:16
dissolved it in water, so you
would say taking your vitamin

377
00:24:16 --> 00:24:20
with a lot of water, and it was
starting to dissolve and being

378
00:24:20 --> 00:24:24
pretty unpleasant, then we
could calculate at equilibrium

379
00:24:24 --> 00:24:28
what kind of p h we would
have in that mixture.

380
00:24:28 --> 00:24:32
Now, these vitamins, this
Nature's Bounty, they do a

381
00:24:32 --> 00:24:36
really good job of isolating
the vitamin, so it is pretty

382
00:24:36 --> 00:24:39
much impossible for
it to dissolve.

383
00:24:39 --> 00:24:41
So they have a nice coating
around it that's highly

384
00:24:41 --> 00:24:47
protective, at least at
normal water p h's, so it

385
00:24:47 --> 00:24:48
doesn't really dissolve.
 

386
00:24:48 --> 00:24:50
And one year I thought I would
do the actual experiment, we

387
00:24:50 --> 00:24:53
could talk about significant
figures, but I could not get

388
00:24:53 --> 00:24:57
the tablet to dissolve -- I
heated it, I stirred it, I did

389
00:24:57 --> 00:25:00
everything at neutral p
h, at room temperature.

390
00:25:00 --> 00:25:02
I want to do it at room
temperature, it just didn't

391
00:25:02 --> 00:25:04
work, even the high
temperature didn't work.

392
00:25:04 --> 00:25:08
So if you buy Nature's Bounty,
it will not dissolve in the

393
00:25:08 --> 00:25:12
water that you're taking, so
just for that little

394
00:25:12 --> 00:25:13
bit of information.
 

395
00:25:13 --> 00:25:16
But if you had, say, an
inferior brand of vitamin C

396
00:25:16 --> 00:25:21
that readily, that didn't have
a nice coating around it, then

397
00:25:21 --> 00:25:23
you could do this experiment.
 

398
00:25:23 --> 00:25:26
So let's take a look at that.
 

399
00:25:26 --> 00:25:29
So the first thing that we have
to do is calculate the molarity

400
00:25:29 --> 00:25:34
of the acid that we've added.
 

401
00:25:34 --> 00:25:36
So here, just have to make
sure that your units are

402
00:25:36 --> 00:25:38
going to be correct.
 

403
00:25:38 --> 00:25:41
So we have grams, we're
converting it with a molecular

404
00:25:41 --> 00:25:45
weight to moles, and then we
have the number of moles in the

405
00:25:45 --> 00:25:49
amount of water, and we can
calculate the molarity

406
00:25:49 --> 00:25:51
of that solution.
 

407
00:25:51 --> 00:25:53
And one of the mistakes that
people often make in doing

408
00:25:53 --> 00:25:56
these problems, they forget to
do all of the conversions that

409
00:25:56 --> 00:26:00
are necessary, sometimes they
stop at moles, and you're

410
00:26:00 --> 00:26:03
talking about concentrations
here, so don't forget about

411
00:26:03 --> 00:26:05
your friend the volume.
 

412
00:26:05 --> 00:26:08
All right, so then you can
write and equation, and I

413
00:26:08 --> 00:26:11
highly recommend that people do
this on the test because it

414
00:26:11 --> 00:26:15
helps them figure out what type
of problem it is and it voids

415
00:26:15 --> 00:26:18
people making silly mistakes.
 

416
00:26:18 --> 00:26:21
So if we're talking about an
acid in water, you should make

417
00:26:21 --> 00:26:24
sure that your equation
reflects an acid in water.

418
00:26:24 --> 00:26:27
If you have hydroxide ion on
the other side, something is

419
00:26:27 --> 00:26:30
very wrong that's going on, and
acid and water is going to be

420
00:26:30 --> 00:26:33
giving hydronium ion
concentrations and

421
00:26:33 --> 00:26:37
a conjugate base.
 

422
00:26:37 --> 00:26:41
So then we can set up an
equilibrium table here.

423
00:26:41 --> 00:26:43
We calculated the
initial molarity.

424
00:26:43 --> 00:26:46
And in the beginning there's
nothing over on this side, so

425
00:26:46 --> 00:26:49
we've just added our
weak acid to water.

426
00:26:49 --> 00:26:53
And so then the change, there's
going to be some amount of this

427
00:26:53 --> 00:26:56
that ionizes minus x, some
amount of this that's formed,

428
00:26:56 --> 00:26:59
and some amount out of the
conjugate that's formed.

429
00:26:59 --> 00:27:05
So we have 0.0284 minus
x plus x plus x.

430
00:27:05 --> 00:27:09
Now we're talking about a weak
acid in water, so what term am

431
00:27:09 --> 00:27:12
I going to want to use next?
 

432
00:27:12 --> 00:27:14
K a.
 

433
00:27:14 --> 00:27:19
So I'm going to want to use k a
next -- k a value is here, 8 .

434
00:27:19 --> 00:27:24
0 times 10 to minus 5, we have
products over our reactant

435
00:27:24 --> 00:27:33
here, and we have x squared
over 0.0284 minus x.

436
00:27:33 --> 00:27:35
Now you can make an assumption
when you're working on these

437
00:27:35 --> 00:27:37
problems and check it later.
 

438
00:27:37 --> 00:27:40
So you can make the assumption
that x is really kind of small

439
00:27:40 --> 00:27:44
compared to this o.084, and
you can just drop this

440
00:27:44 --> 00:27:47
x out of the term here.
 

441
00:27:47 --> 00:27:51
And then check later and
see if that worked or not.

442
00:27:51 --> 00:27:56
So that makes the math easier,
and so now we can just solve

443
00:27:56 --> 00:28:00
for x, and x comes
out to be 0.00151.

444
00:28:00 --> 00:28:06
The really two significant
figures, but we're going to

445
00:28:06 --> 00:28:10
carry an extra one for the
moment, so they're just two

446
00:28:10 --> 00:28:14
figures right here, two
significant figures here.

447
00:28:14 --> 00:28:17
Now we can check and see if
x was really small, if that

448
00:28:17 --> 00:28:18
assumption worked right.
 

449
00:28:18 --> 00:28:27
So, is 0.0284 minus 0.00151
really the same as 0.0248, and

450
00:28:27 --> 00:28:30
we let you make the assumption
that it is, we say it's OK if

451
00:28:30 --> 00:28:34
it's less than 5% of the value.
 

452
00:28:34 --> 00:28:37
So, in this case, it's
actually not, it's 5 .

453
00:28:37 --> 00:28:41
3, so that violates our policy.
 

454
00:28:41 --> 00:28:45
So it's more than 5%, so then
you have to use the quadratic

455
00:28:45 --> 00:28:48
equation to solve the problem.
 

456
00:28:48 --> 00:28:52
I just want to note that this
term, this percentage, can be

457
00:28:52 --> 00:28:54
called sometimes percent
ionized or percent

458
00:28:54 --> 00:28:57
deprotonated, so that you're
not thinking that's some

459
00:28:57 --> 00:29:01
kind of bizarre term
if you see that.

460
00:29:01 --> 00:29:04
And if you use the quadratic
equation, you get an answer of

461
00:29:04 --> 00:29:08
0.00147, again, that's really
two significant figures.

462
00:29:08 --> 00:29:11
So it's not a whole lot
different, actually, than

463
00:29:11 --> 00:29:16
the number you got making
the approximation.

464
00:29:16 --> 00:29:21
So, once you know what x is, x
is the hydronium ion

465
00:29:21 --> 00:29:24
concentration in this problem,
and so we can plug that in,

466
00:29:24 --> 00:29:28
p h is minus log of
this, so that's 2 .

467
00:29:28 --> 00:29:30
83.
 

468
00:29:30 --> 00:29:35
And so we had really two
significant figures here, and

469
00:29:35 --> 00:29:38
so we are going to have two
significant figures after

470
00:29:38 --> 00:29:40
the decimal point here.
 

471
00:29:40 --> 00:29:42
And if you haven't reviewed
your sig fig rules and need

472
00:29:42 --> 00:29:46
more help if that seems wrong,
then you should definitely

473
00:29:46 --> 00:29:47
review it before the next test.
 

474
00:29:47 --> 00:29:55
All right, so now we'll
continue with today's lecture

475
00:29:55 --> 00:29:58
notes, and we're just going to
continue right on and we're

476
00:29:58 --> 00:30:03
going to talk about weak bases,
and well, we're start working

477
00:30:03 --> 00:30:05
on our way through, we're also
going to try to get

478
00:30:05 --> 00:30:06
to buffers today.
 

479
00:30:06 --> 00:30:10
So, we've done a problem
for a weak acid in water.

480
00:30:10 --> 00:30:19
So now let's talk about a
weak base in water, and

481
00:30:19 --> 00:30:20
you can start us off.
 

482
00:30:20 --> 00:30:25
So in this problem we're given
a molarity, so you didn't have

483
00:30:25 --> 00:30:29
to calculate that, and now you
can help me fill out the table

484
00:30:29 --> 00:31:15
so we know what to do here.
 

485
00:31:15 --> 00:31:30
OK, let's just do
10 more seconds.

486
00:31:30 --> 00:31:33
Very good.
 

487
00:31:33 --> 00:31:38
So you're going to be losing
some of the amount -- some of

488
00:31:38 --> 00:31:40
the amount of the weak base you
have in, it's going to ionize,

489
00:31:40 --> 00:31:45
and so then you'll be forming
the conjugate acid plus x, and

490
00:31:45 --> 00:31:48
you're going to be forming
hydroxide ions plus x.

491
00:31:48 --> 00:31:51
And so the one with the minus
sign is important, and

492
00:31:51 --> 00:31:54
sometimes there will be 2's
involved, and that depends on

493
00:31:54 --> 00:31:57
the stoichiometry
of the reaction.

494
00:31:57 --> 00:32:03
All right, so we can use that
information now and go on and

495
00:32:03 --> 00:32:06
look at -- actually you could
leave that clicker question

496
00:32:06 --> 00:32:11
up for a minute, and we're
going to talk about the k b.

497
00:32:11 --> 00:32:17
So the k b is going to be
equal to our products, n h 4

498
00:32:17 --> 00:32:25
plus and our hydroxide ion
concentration over n h 3.

499
00:32:25 --> 00:32:30
And so now I can fill in the
values that you told me.

500
00:32:30 --> 00:32:34
So we have, on the top we're
going to have x squared, on

501
00:32:34 --> 00:32:38
the bottom we're going
to have 0.15 minus x.

502
00:32:38 --> 00:32:44
And now we can make an
approximation here that x is

503
00:32:44 --> 00:32:48
going to be small compared to
0.15, and so we can say that's

504
00:32:48 --> 00:32:54
just going to be equal to x
squared over 0.15, and the k b

505
00:32:54 --> 00:32:56
value that was given was 1 .
 

506
00:32:56 --> 00:33:02
8 times 10 to the minus 5.
 

507
00:33:02 --> 00:33:08
So now we can solve for x using
this approximation, and using

508
00:33:08 --> 00:33:11
this approximation x
comes out to be 0 .

509
00:33:11 --> 00:33:18
00164, and we can
look at whether that

510
00:33:18 --> 00:33:20
approximation was OK.
 

511
00:33:20 --> 00:33:31
So is this number less than 5%
of 0.15, so we can say 0.00164

512
00:33:31 --> 00:33:40
over 0.15 times 100, and that
comes out to be 1.1%, so that's

513
00:33:40 --> 00:33:43
OK, that's less than 5%.
 

514
00:33:43 --> 00:33:46
So that's good, we
don't have to use the

515
00:33:46 --> 00:33:50
quadratic equation here.
 

516
00:33:50 --> 00:33:56
So now, we want to
calculate the p h.

517
00:33:56 --> 00:34:03
So can I just plug that number
for x into my p h equation?

518
00:34:03 --> 00:34:05
What is x?
 

519
00:34:05 --> 00:34:13
What is x equal to here?
 

520
00:34:13 --> 00:34:14
It's equal to two
different things.

521
00:34:14 --> 00:34:17
What's one of them?
 

522
00:34:17 --> 00:34:20
Hydroxide ion concentration.
 

523
00:34:20 --> 00:34:23
So what we can do is
calculate a p o h.

524
00:34:23 --> 00:34:29
So, p o h is minus log of the
hydroxide ion concentration,

525
00:34:29 --> 00:34:34
or minus log of 0 .
 

526
00:34:34 --> 00:34:41
001647, and we really only
have two significant figures

527
00:34:41 --> 00:34:45
here, and that is going
to come out to 2 .

528
00:34:45 --> 00:34:48
79.
 

529
00:34:48 --> 00:34:50
And so, we would have two
significant figures after the

530
00:34:50 --> 00:34:53
decimal point, because this
number had two significant

531
00:34:53 --> 00:34:54
figures in it.
 

532
00:34:54 --> 00:34:55
But I'm not done.
 

533
00:34:55 --> 00:34:59
I've calculated p o h and
the problem wanted p h.

534
00:34:59 --> 00:35:05
So how do I go from
p o h to p h?

535
00:35:05 --> 00:35:08
14 minus, yup.
 

536
00:35:08 --> 00:35:10
So 14 .
 

537
00:35:10 --> 00:35:14
00 at room temperature
minus 2 .

538
00:35:14 --> 00:35:19
79 is going to be equal to 11 .
 

539
00:35:19 --> 00:35:23
21.
 

540
00:35:23 --> 00:35:24
And so that makes sense.
 

541
00:35:24 --> 00:35:28
Now in doing these problems
always consider, sometimes

542
00:35:28 --> 00:35:31
you're rushing and you get
done you say OK, my p h is 2.

543
00:35:31 --> 00:35:35
But go back and think about the
type of problem you're doing.

544
00:35:35 --> 00:35:37
It's a base in water problem.
 

545
00:35:37 --> 00:35:40
Would it make sense that
the p h was 2 if it was

546
00:35:40 --> 00:35:41
a base in water problem?
 

547
00:35:41 --> 00:35:42
No.
 

548
00:35:42 --> 00:35:46
And so then you realize oh,
I have to do another step.

549
00:35:46 --> 00:35:50
So that kind of thinking can
save you a lot of points on the

550
00:35:50 --> 00:35:53
exam, to remember what it is
you're trying to calculate and

551
00:35:53 --> 00:35:57
go back and make sure that
your answer makes sense.

552
00:35:57 --> 00:35:59
And sometimes people run into
weird math problems and some

553
00:35:59 --> 00:36:05
they'll write and say this p h
should be above 7, it's 2, I

554
00:36:05 --> 00:36:08
don't know what I did wrong,
clearly I did something wrong.

555
00:36:08 --> 00:36:10
I know that's wrong but I
don't have time to figure

556
00:36:10 --> 00:36:12
out what I did wrong.
 

557
00:36:12 --> 00:36:13
That will get you points.
 

558
00:36:13 --> 00:36:17
So just recognizing that if
something makes sense or not

559
00:36:17 --> 00:36:20
tells us you know what's going
on, and sometimes math issues

560
00:36:20 --> 00:36:23
can get you into a place that
you can't get out of quickly.

561
00:36:23 --> 00:36:26
So just thinking about
whether the problem makes

562
00:36:26 --> 00:36:27
sense is a big step.
 

563
00:36:27 --> 00:36:30
All right.
 

564
00:36:30 --> 00:36:34
So now, we're going to talk
about salt problems and I'm

565
00:36:34 --> 00:36:39
going to try to convince you
that salts are actually the

566
00:36:39 --> 00:36:44
same as the weak acid and
weak bases that we just did.

567
00:36:44 --> 00:36:47
So a salt is formed when
you mix an acid and

568
00:36:47 --> 00:36:49
a base together.
 

569
00:36:49 --> 00:36:53
So, for example, if you have
h c l and sodium hydroxide,

570
00:36:53 --> 00:36:58
you're going to get table
salt and a c l and water.

571
00:36:58 --> 00:37:04
So the p h of a salt in water
is not always neutral.

572
00:37:04 --> 00:37:07
Sometimes it's neutral,
sometimes it's not neutral.

573
00:37:07 --> 00:37:09
Well, when would it
not be neutral?

574
00:37:09 --> 00:37:13
Well, if a salt contained a
conjugate acid of a weak base,

575
00:37:13 --> 00:37:18
then that conjugate acid is
going to make it weakly acidic.

576
00:37:18 --> 00:37:24
Salts that contain things like
iron 3 plus also may be acidic.

577
00:37:24 --> 00:37:28
So when you're drinking water
and you measure the p h of that

578
00:37:28 --> 00:37:31
water and it's not neutral,
this could be part of the

579
00:37:31 --> 00:37:34
reason that there's some salt
in the water, some different

580
00:37:34 --> 00:37:36
ions in the water.
 

581
00:37:36 --> 00:37:40
So a general rule from the
periodic table group one and

582
00:37:40 --> 00:37:45
group two metals, so lithium,
calcium, sodium, those are all

583
00:37:45 --> 00:37:47
going to be neutral in
solution, so you can

584
00:37:47 --> 00:37:48
just remember that.
 

585
00:37:48 --> 00:37:52
And if a salt contains a
conjugate base of a weak

586
00:37:52 --> 00:37:55
acid, then it'll form
a basic solution.

587
00:37:55 --> 00:37:59
So it's all about whether the
salt derived from a weak acid

588
00:37:59 --> 00:38:02
or a weak base that's going to
give you a clue as whether it's

589
00:38:02 --> 00:38:04
an acid or a basic solution.
 

590
00:38:04 --> 00:38:08
If it derived, say, from a
strong acid mixed with a strong

591
00:38:08 --> 00:38:11
base, then it's going to give
you a salt that's neutral.

592
00:38:11 --> 00:38:13
So let's look at some examples.
 

593
00:38:13 --> 00:38:19
So here we have n h 4 c l.
 

594
00:38:19 --> 00:38:22
So we can break this down and
think about where a salt like

595
00:38:22 --> 00:38:25
this would have come from.
 

596
00:38:25 --> 00:38:28
So it would have come from
n h 4 plus, and it would

597
00:38:28 --> 00:38:32
have come from c l minus.
 

598
00:38:32 --> 00:38:36
So n h 4 plus, let's think
about where this came from.

599
00:38:36 --> 00:38:38
What is it?
 

600
00:38:38 --> 00:38:43
So, we want to ask the question
is n h 4 plus a conjugate

601
00:38:43 --> 00:38:45
acid of a weak base?
 

602
00:38:45 --> 00:38:47
And what is its conjugate base?
 

603
00:38:47 --> 00:38:50
Well, it's conjugate
base is n h 3.

604
00:38:50 --> 00:38:53
If you lose a hydrogen
ion or proton from n h

605
00:38:53 --> 00:38:55
4 plus, you get n h 3.
 

606
00:38:55 --> 00:38:58
And so you're really asking, if
that's a weak base, then its

607
00:38:58 --> 00:39:00
conjugate is also
going to be weak.

608
00:39:00 --> 00:39:03
So you need to know about these
guys to see what would happen

609
00:39:03 --> 00:39:06
if you have n h 4
plus in solution.

610
00:39:06 --> 00:39:08
So how do you know about this.
 

611
00:39:08 --> 00:39:12
Well, you know about things
being weak or strong based on

612
00:39:12 --> 00:39:14
their k a's and their k b's.
 

613
00:39:14 --> 00:39:17
So, is ammonia a weak base?
 

614
00:39:17 --> 00:39:19
It has a k b of 1 .
 

615
00:39:19 --> 00:39:24
8 times 10 to the minus 5, so
yeah, that's a small number,

616
00:39:24 --> 00:39:26
so that's a weak base.
 

617
00:39:26 --> 00:39:30
And so it's actually in this
table, so if you have something

618
00:39:30 --> 00:39:34
that's weak over here, then its
conjugate is also going

619
00:39:34 --> 00:39:35
to be weak over there.
 

620
00:39:35 --> 00:39:37
These are totally
lined up right.

621
00:39:37 --> 00:39:42
And so the conjugate over here
is also going to be weak.

622
00:39:42 --> 00:39:45
And if you weren't really sure
you could always look it up, so

623
00:39:45 --> 00:39:49
here you have ammonium ion
and it has a k a of 5 .

624
00:39:49 --> 00:39:51
6 times 10 to the minus 10.
 

625
00:39:51 --> 00:39:56
Yup, that's a very small
number, that is a weak acid.

626
00:39:56 --> 00:40:02
So yes, the conjugate is weak,
the base is weak, and the

627
00:40:02 --> 00:40:05
conjugate acid of that
weak base is also weak.

628
00:40:05 --> 00:40:09
So, n h 4 plus does have
acidic properties.

629
00:40:09 --> 00:40:12
It's not a strong acid, it's
a weak acid, but it will

630
00:40:12 --> 00:40:15
make things acidic.
 

631
00:40:15 --> 00:40:17
So this should be acidic.
 

632
00:40:17 --> 00:40:19
What about c l minus?
 

633
00:40:19 --> 00:40:23
Do you think it's going to
do anything useful for you?

634
00:40:23 --> 00:40:27
Where do you think c
l minus came from?

635
00:40:27 --> 00:40:29
From h c l.
 

636
00:40:29 --> 00:40:32
So we could ask again, is
a c l minus a conjugate

637
00:40:32 --> 00:40:34
base of a weak acid.
 

638
00:40:34 --> 00:40:38
The acid is h c l, is
that a weak acid?

639
00:40:38 --> 00:40:39
No.
 

640
00:40:39 --> 00:40:44
So we can look it up if you
didn't remember it, 10 to the

641
00:40:44 --> 00:40:49
7, definitely not weak -- very,
very, very strong acid, and

642
00:40:49 --> 00:40:53
if something is strong
acid, its conjugate is

643
00:40:53 --> 00:40:55
ineffective as a base.
 

644
00:40:55 --> 00:40:59
So c l minus is ineffective
as a base so it's going

645
00:40:59 --> 00:41:02
to be neutral here.
 

646
00:41:02 --> 00:41:05
So overall, you have something
that's going to be acidic

647
00:41:05 --> 00:41:07
with something that's
going to be neutral.

648
00:41:07 --> 00:41:09
So overall it'll be acidic.
 

649
00:41:09 --> 00:41:13
So this particular salt and
water is going to be acidic

650
00:41:13 --> 00:41:16
because the things that were
mixed together to get it, one

651
00:41:16 --> 00:41:19
of them included a weak base,
and so that's going to form a

652
00:41:19 --> 00:41:25
weak conjugate acid, so it'll
be acidic in solution.

653
00:41:25 --> 00:41:27
All right, so let's
look at another one.

654
00:41:27 --> 00:41:33
I'm giving you the k a value in
this problem, and knowing that

655
00:41:33 --> 00:41:37
particular k a value, tell me
what you think is going to be

656
00:41:37 --> 00:41:41
true about this particular salt
in solution, whether it'll be

657
00:41:41 --> 00:42:27
acidic, neutral, or basic.
 

658
00:42:27 --> 00:42:46
OK, 10 more seconds.
 

659
00:42:46 --> 00:42:49
So, some people were a little
fooled by the information

660
00:42:49 --> 00:42:53
I gave you, so let's
take a look at this.

661
00:42:53 --> 00:43:00
So if we go to my
presentation here.

662
00:43:00 --> 00:43:07
So we can break this up into n
a plus and c h 3 c o minus.

663
00:43:07 --> 00:43:13
N a plus, is that a conjugate
acid of a weak base?

664
00:43:13 --> 00:43:14
Is that going to be acidic?
 

665
00:43:14 --> 00:43:18
What do we know about things
in group one, a column

666
00:43:18 --> 00:43:20
of the periodic table.
 

667
00:43:20 --> 00:43:22
They're going to be neutral.
 

668
00:43:22 --> 00:43:26
Where do you think that this
came from, n a plus, where

669
00:43:26 --> 00:43:30
might it have come from?
 

670
00:43:30 --> 00:43:32
It might have come from n a
o h, that might have been

671
00:43:32 --> 00:43:34
a base that was added.
 

672
00:43:34 --> 00:43:36
So it's not going to do
anything for you -- things

673
00:43:36 --> 00:43:39
in group one and group two
are going to be neutral.

674
00:43:39 --> 00:43:42
All right, so we can ask
the question about a

675
00:43:42 --> 00:43:45
c h 3 c o o minus.
 

676
00:43:45 --> 00:43:49
Is it a conjugate
base of a weak acid?

677
00:43:49 --> 00:43:53
So then we can say is the acid
that it came from a weak acid?

678
00:43:53 --> 00:43:55
Is its conjugate a weak acid?
 

679
00:43:55 --> 00:43:57
Well, how do we
know about that?

680
00:43:57 --> 00:44:01
Well, we know about that from
the k a value that I gave you.

681
00:44:01 --> 00:44:04
So is this a weak acid?
 

682
00:44:04 --> 00:44:05
Yes.
 

683
00:44:05 --> 00:44:09
So is its conjugate going
to be a weak base?

684
00:44:09 --> 00:44:11
Yes.
 

685
00:44:11 --> 00:44:14
So, given that information
we can say yes.

686
00:44:14 --> 00:44:17
So its conjugate acid
is weak, so then it

687
00:44:17 --> 00:44:19
would be a weak base.
 

688
00:44:19 --> 00:44:25
So if it's a weak base then
it'll be basic in solution,

689
00:44:25 --> 00:44:28
and we have something that's
neutral plus basic, so

690
00:44:28 --> 00:44:30
overall you get basic.
 

691
00:44:30 --> 00:44:34
All right.
 

692
00:44:34 --> 00:44:45
So let's look at a general
example now of this as well.

693
00:44:45 --> 00:44:48
So here we can talk
about a general rule.

694
00:44:48 --> 00:44:53
So if you have compound
x y, we can talk about

695
00:44:53 --> 00:44:55
x plus and y minus.
 

696
00:44:55 --> 00:44:58
And for x plus you're going
to be asking about is it a

697
00:44:58 --> 00:45:01
conjugate acid of a weak base.
 

698
00:45:01 --> 00:45:04
For y minus you're going to
be asking is it a conjugate

699
00:45:04 --> 00:45:05
base of a weak acid.
 

700
00:45:05 --> 00:45:08
So for the first part you're
asking about if it's a

701
00:45:08 --> 00:45:11
conjugate acid, in the second
half you're asking about if

702
00:45:11 --> 00:45:13
it's a conjugate base.
 

703
00:45:13 --> 00:45:17
So if something is a conjugate
acid of a weak base, and that's

704
00:45:17 --> 00:45:19
yes -- if you know that's a
weak base or you know that it's

705
00:45:19 --> 00:45:23
a weak acid, then it's going to
be acidic, if it's not,

706
00:45:23 --> 00:45:24
it'll be neutral.
 

707
00:45:24 --> 00:45:27
Same thing is true over here.
 

708
00:45:27 --> 00:45:29
You might know about that
something is a weak base, you

709
00:45:29 --> 00:45:32
might know that its conjugate
is a weak acid, and if you have

710
00:45:32 --> 00:45:35
a conjugate base of something
that's a weak acid,

711
00:45:35 --> 00:45:36
then it is a base.
 

712
00:45:36 --> 00:45:39
If it's a strong acid it would
be ineffectual, but if it's

713
00:45:39 --> 00:45:42
a conjugate of a weak
acid, it'll be basic.

714
00:45:42 --> 00:45:44
No, it's neutral.
 

715
00:45:44 --> 00:45:47
So overall, you can have
three possibilities.

716
00:45:47 --> 00:45:51
Acidic plus neutral is acidic,
basic plus neutral is

717
00:45:51 --> 00:45:54
basic, and neutral plus
neutral is neutral.

718
00:45:54 --> 00:45:57
Now some people might come up
with another option here.

719
00:45:57 --> 00:46:03
What other thing am I leaving
off of this overall?

720
00:46:03 --> 00:46:04
Acidic plus basic.
 

721
00:46:04 --> 00:46:06
That's because I'm never
going to ask you that

722
00:46:06 --> 00:46:07
when it comes to a salt.
 

723
00:46:07 --> 00:46:09
Because pretty much, salts are
formed when you're doing a

724
00:46:09 --> 00:46:12
titration, and you're going to
be either titrating a strong

725
00:46:12 --> 00:46:15
acid, a strong base, you're
going to be titrating a weak

726
00:46:15 --> 00:46:19
acid with a strong base, or
you're going to be titrating a

727
00:46:19 --> 00:46:21
strong acid with a weak base.
 

728
00:46:21 --> 00:46:25
You are never going to titrate
a weak acid with a weak base.

729
00:46:25 --> 00:46:28
That would yield no interesting
results of any kind.

730
00:46:28 --> 00:46:30
So you're not going to be
forming salts that are

731
00:46:30 --> 00:46:32
conjugates of both
those things.

732
00:46:32 --> 00:46:35
So, if you want to think about
it that way that's fine, or you

733
00:46:35 --> 00:46:37
could just remember that
that is not what I'm

734
00:46:37 --> 00:46:38
going to ask you.
 

735
00:46:38 --> 00:46:42
These are great little short
answer questions on an exam, so

736
00:46:42 --> 00:46:44
if you're good at thinking
about this, it'll definitely

737
00:46:44 --> 00:46:51
give you a couple of points
on one of the exams.

738
00:46:51 --> 00:46:54
All right, so now and last,
just a couple minutes, I

739
00:46:54 --> 00:46:58
just want to introduce
very briefly buffers.

740
00:46:58 --> 00:47:02
So a buffer is something that
maintains the p h of a

741
00:47:02 --> 00:47:06
solution, so it's going
to buffer that solution.

742
00:47:06 --> 00:47:10
So if you add a little bit of
strong acid or a little bit of

743
00:47:10 --> 00:47:12
strong base, it doesn't
matter the p h is going

744
00:47:12 --> 00:47:13
to stay the same.
 

745
00:47:13 --> 00:47:16
So there are two
kinds of buffers.

746
00:47:16 --> 00:47:20
You have an acid buffer, which
is going to buffer/maintain the

747
00:47:20 --> 00:47:23
p h on the acidic
side of neutral.

748
00:47:23 --> 00:47:27
And a basic buffer, which
will maintain the p h on the

749
00:47:27 --> 00:47:31
basic end of the p h scale.
 

750
00:47:31 --> 00:47:35
So let me just give you a brief
example of a buffer and just

751
00:47:35 --> 00:47:37
get you thinking about buffers.
 

752
00:47:37 --> 00:47:40
So here, in about a buffer
problem you're going to mix an

753
00:47:40 --> 00:47:43
acid with its conjugate base.
 

754
00:47:43 --> 00:47:48
So, you have acetate, and
then probably the acetate

755
00:47:48 --> 00:47:50
salt of the acetic acid.
 

756
00:47:50 --> 00:47:53
So over here, you have the
acetic acid, on this side you

757
00:47:53 --> 00:47:57
have its conjugate base usually
added in the form of a salt.

758
00:47:57 --> 00:48:00
And then you have
an equilibrium.

759
00:48:00 --> 00:48:03
So, what's going to happen
if you add a strong

760
00:48:03 --> 00:48:06
acid to this solution?
 

761
00:48:06 --> 00:48:13
If you add strong acid, if you
add more h 3 o plus, what

762
00:48:13 --> 00:48:14
happens if you add more?
 

763
00:48:14 --> 00:48:19
What direction will
the reaction shift?

764
00:48:19 --> 00:48:22
So you'll get the back
reaction, it'll try to minimize

765
00:48:22 --> 00:48:26
that stress and move the other
way, and it'll use up some of

766
00:48:26 --> 00:48:29
that acid and maintain the p h.
 

767
00:48:29 --> 00:48:33
Then you can think, so these
amount of acid added is

768
00:48:33 --> 00:48:37
effectively removed and
the p h stays the same.

769
00:48:37 --> 00:48:41
What about if you
add a strong base?

770
00:48:41 --> 00:48:47
Well, that strong base will
react with the acid, it will

771
00:48:47 --> 00:48:53
remove protons from this acid,
or the hydrogen ion here,

772
00:48:53 --> 00:48:55
forming this water
and its conjugate.

773
00:48:55 --> 00:48:57
So you'll make more of
these and the p h will

774
00:48:57 --> 00:48:58
also stay the same.
 

775
00:48:58 --> 00:49:01
So the base is going to
be removed by reacting.

776
00:49:01 --> 00:49:06
So they're effectively removed
and the p h stays the same.

777
00:49:06 --> 00:49:10
So in this you have a weak
acid, h a, it'll transfer

778
00:49:10 --> 00:49:17
protons to o h minus supplied
by the strong base.

779
00:49:17 --> 00:49:21
The conjugate of that weak
acid, which would be a weak

780
00:49:21 --> 00:49:25
base, is going to accept
protons from any acid that

781
00:49:25 --> 00:49:27
is going to be supplied.
 

782
00:49:27 --> 00:49:32
So, in this way, you
maintain the p h.

783
00:49:32 --> 00:49:38
And so I just to emphasize that
in a buffer solution you have h

784
00:49:38 --> 00:49:42
a, acidic buffer solution,
you have h a, and you

785
00:49:42 --> 00:49:43
have its conjugate.
 

786
00:49:43 --> 00:49:46
And if you only have one or
the other, it's not going

787
00:49:46 --> 00:49:47
to make a good buffer.
 

788
00:49:47 --> 00:49:52
So, I want you to remember that
in buffers you have both

789
00:49:52 --> 00:49:55
conjugates -- one alone
is not going to work.

790
00:49:55 --> 00:49:59
And people forget this in the
class, and so you can remember

791
00:49:59 --> 00:50:02
that for Halloween, your
chemistry professor dressed

792
00:50:02 --> 00:50:07
up as a buffer to help you
remember that in a buffer

793
00:50:07 --> 00:50:09
you're going to have both.
 

794
00:50:09 --> 00:50:12
You've got to have the
conjugate acid base set,

795
00:50:12 --> 00:50:15
otherwise it will not buffer.