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## 2015 AP Chemistry free response questions

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# 2015 AP Chemistry free response 3e

## Video transcript

- The initial pH and the equivalence point are plotted on the graph below. Accurately sketch the titration
curve on the graph below. Mark the position of the
half-equivalence point on the curve with an X. All right, so we have, they show us the initial pH right over
here, and they also show us the equivalence point right over here. And it's at a pH of, we actually saw that in the last problem with equivalence, the equivalence point, or
actually a couple of problems ago, the equivalence point was measured, the pH at the equivalence
point was measured to be 2.54, so that's why this right
over here is at 2.54 when we've added about 29 point, I forgot what the number was. It was in, I think, the
second part of this problem. They said when you added
close to 30 milliliters of that hydrochloric acid, that 1.25 molar hydrochloric acid solution, that is when we hit our equivalence point. And we see the pH right over there, and then the half-equivalence point. Well, that's going to be when
we've added half this amount. So it's going to be a
little bit less than 15, and we in the last problem, we figured out that that was going to be at a pH of 4.77. So a pH of 4.77 is going
to be something like, right over there, they
said mark that with an X. So half-equivalence
point, we've added half the titrate there, and we know the pH. We figured that out in the
last part of the problem. And now we just need to graph, we need to sketch the titration curve. So let's see. We keep adding more and
more hydrochloric acid. Hydrochloric acid, it lowers the pH. We get to the half-equivalence point. And then actually, we're going to start leveling off over here. The reason why you level off
is 'cause you're going to have, you're going to have more and
more of the conjugate acid there, and so as you react
with the conjugate base, with more and more of the conjugate base, the equilibrium between the conjugate base and the conjugate acid,
the more of that acid is going to go to conjugate base. So you're going to have a
little bit of a buffering going on, but at some
point you have reacted. You have completely
reacted with everything, and you've hit your equivalence point. You hit your equivalence point. And then, and you become much more acidic, and it might look something, it might look something like that. Let me see if I can do
a little bit better, a little bit better job. So it would look something, and we're obviously, it's not
going to be exactly right. It's important we go
through these three points. And it would look something like that. So that's the equivalence point, half-equivalence point, and
then this is our initial pH.