2015 AP Chemistry free response 3e | Chemistry | Khan Academy

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. At the pH of— we actually saw that in the last problem with the equivalence. 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 the— it was in, I think, the second part of this problem. They said when you added close to 30 milliliters of that hydrochloric acid, of 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. In the last problem, we figured out that that was going to be a pH of 4.77. So a pH of 4.77 is going to be something like right over there. They said to mark that with an X. So the half equivalence point— we've added half the titrant there, and we know the pH. We figured that out in the last part of the problem.

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 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 because you're going to have more and more of the conjugate acid there.

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— more of that acid is going to go to conjugate base, so you're going to have a little bit of buffering going on. But at some point, you have completely reacted with everything, and you hit your equivalence point. You hit your equivalence point, and then you've become much more acidic.

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.