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# 2015 AP Chemistry free response 3f

Analyzing concentrations when pH lower than half-equivalence point. From 2015 AP Chemistry free response 3f.

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• Shoudn't the conjugate base have a higher concentration, as most of the sorbic acid molecules has lost its acidic proton to lower the pH?
• I can think of it the following way:
Initially, we had only sorbate ions, as potassium sorbate dissociates completely. Then some of sorbate ions picked hydrogen from HCl forming weak sorbic acid, small part of which dissociated again, this time making hydronium and actually lowering pH. Since at the half equivalence point we have the same amount of sorbic acid as sorbate ions (but not the hydronium, as sorbic acid is weak), then, as pH went down from equivalence point, more sorbic acid was made, leading to more - > more hydrogen ions were released.
It wouldn't be correct to assume that most of the sorbic acid lost its protons, as it's Ka is low: 1.7x10^-5, and for losing most of the protons it should be much higher than 1.
• If you wanted to prove this mathematically, couldn't you simply use the Henderson-Hasselbalch formula as you have pKa for sorbic acid from the original question (shown in previous videos) and use the value of the log of ratio of concentrations to determine which is in higher concentration?
• Yes, this gives you:`[sorbate] / [sorbic acid] = 0.040`.
• Why is there equal concentrations of sorbate and sorbic acid at the half equivalence point and not the equivalence point?
• At the equivalence point ALL the acid has been used up and so ALL of it has been converted into the conjugate base.

At the HALF equivalence point HALF the acid has been used up so HALF of it has been converted into the conjugate base.
• The question is asking about a pH which is between the half equivalence point (pH=4.77) and the equivalence point (pH=2.7, approximated from the graph). The main reaction at this point is C6H7O2- and H+ reacting to form HC6H7O2. I was wondering if I can use the fact to justify that because the acid dissociation constant for HC6H7O2 (sorbic acid) is small, Ka=1.7x10^-5 (HC6H7O2 <--> C6H7O2- + H+), the reverse reaction is favored, meaning that there will be more HC6H7O2?
• In the Above SET of Problems, Can we calculate pH at Equivalence Point? How? Will we consider Sorbic acid at Equivalence point as Weak Acid and then calculate its pH??
(1 vote)