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Course: Chemistry library > Unit 14
Lesson 2: Titrations- Acid–base titrations
- Worked example: Determining solute concentration by acid–base titration
- Titration of a strong acid with a strong base
- Titration of a strong acid with a strong base (continued)
- Titration of a weak acid with a strong base
- Titration of a weak acid with a strong base (continued)
- Titration of a weak base with a strong acid
- Titration of a weak base with a strong acid (continued)
- 2015 AP Chemistry free response 3b
- 2015 AP Chemistry free response 3c
- 2015 AP Chemistry free response 3d
- 2015 AP Chemistry free response 3e
- 2015 AP Chemistry free response 3f
- Titration curves and acid-base indicators
- Redox titrations
- Introduction to titration
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2015 AP Chemistry free response 3e
Sketching a titration curve based on half-equivalence and equivalence points. From 2015 AP Chemistry free response 3e.
Want to join the conversation?
- Why would the pH continue to decrease after reaching the equivalence point?(5 votes)
- Why wouldn't it? If you continue to add acid past the equivalence point you're increasing the concentration of H+ so pH will get lower and lower(6 votes)
- Why the pH value of equivalence point is not 7?(1 vote)
- When all the sorbic acid is reacted it forms sorbate which is a weak base and will react with water to produce some OH- making the pH greater than 7
Only in the case with a strong acid and strong base will the equivalence point be at pH = 7
Weak acid + strong base -> pH > 7
Strong acid + weak base -> pH < 7
Weak acid + weak base -> depends on which is weaker(3 votes)
- Shouldnt the X be in the middle of the plateau? In the middle of the buffer region I mean.(1 vote)
- He has it at the correct volume and pH, check the data on the previous questions, he’s just drawn the graph a bit wrong.(1 vote)
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.