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

The pH of the soft drink is 3.37. After the addition of the potassium sorbate, which species, the sorbic acid or the sorbate ion, has a higher concentration in the soft drink? Justify your answer.

So, this is related to the question we've been doing because the sorbate ion is what happens when you put potassium sorbate in a solution; it dissociates. The concentration of the sorbate is the same as the concentration of potassium sorbate.

One way to think about it is that it is going to be the same as the concentration of your potassium sorbate. If we think about titrating potassium sorbate, at what point do you have an equal concentration of potassium sorbate and sorbic acid? You have equal concentrations at the half equivalence point.

We marked that out when we figured it out in the last few parts of this problem. The half equivalence point happens at a pH of 4.77. We could say the half equivalence point of titration of potassium sorbate with hydrochloric acid happens at a pH of 4.77.

At this point, you have equal concentrations of C6H7O2⁻ and H C6H7O2. The pH of 3.37 is lower and will thus occur beyond the half equivalence point. So, the concentration of the sorbic acid is higher.

When you're starting off with a titration, your concentration of potassium sorbate will be higher. Then, as you keep titrating, it reacts with the hydrochloric acid, reaching the half equivalence point where these two will have equal concentrations.

If you keep titrating, it becomes more acidic, and you will have a higher concentration of the sorbic acid. One way to think about this is to underline that whole part: the concentration of sorbic acid is higher. That’s the main thing they’re asking for.

But the way to think about this soft drink is that it might not obviously show people titrating every soft drink. You could view this soft drink as having reached that point after a theoretical titration that brought us past the half equivalence point. Therefore, most of what we care about is in its acid form versus the conjugate base.

Most of the sorbic acid is in its acid form, or I guess you could say the other way: most of the sorbate has become the conjugate acid.