I got AP Chem curriculum from a teacher who used to grade the actual test, through his protege, so to speak. I’ve been over this with a tutor a couple times, so I’m not just jumping over here to be lazy. It’s only my 2nd year of AP, and some of this stuff hasn’t come up since I took college chem 20 years ago.
Potassium sorbate, KC6H7O2 (molar mass 150. g/mol) is commonly added to diet soft drinks as a preservative. A stock solution of KC6H7O2(aq) of known concentration must be prepared. A student titrates 45.00 mL of the stock solution with 1.25 M HCl(aq) using both an indicator and a pH meter. The value of Ka for sorbic acid, HC6H7O2, is 1.7 × 10−5.
(e) 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.
Here’s the answer given, but the line should be shifted a bit left to land on the dots. http://i86.photobucket.com/albums/k110/flavord/AP%20Chem%205-11%20graph%20practice%20test_zpsr6mdfbyq.jpg
Here’s my current explanation of the graph, as far as I have one. Please poke holes in this at any point:
The HCl releases the protons essentially 100%. At first, the molecule most available to take them is water, so the hydronium concentration spikes, which means the pH is falling fast. After a bit, there is so much hydronium that it meets the sorbate ion in quantities high enough to slow the pH fall very noticeably. After another bit of time/added HCl, enough of the sorbate is protonated so that some of it is even putting protons onto water, after all, it’s a weak acid, and water is a better base than it is. The hydronium concentration grows rapidly, and the pH falls sharply.
If that all is correct above, I still don’t understand why there’s the tail that starts to flatten out to the right of the equivalence point. We’re still adding HCl, but the sorbate ion is apparently taking the proton quite a bit there. Is it basically Le Chatelier’s Principle, that if we load up the protonated side, the system shifts to the other side, and forms sorbic acid?