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# 2015 AP Chemistry free response 3d

Calculating the pH at the half-equivalence point for the titration of a weak base with HCl. From 2015 AP Chemistry free response 3d.

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• At , the video mentioned that at the half-equivalence point, the [A-] and [HA] are equivalent. Is this only true when one mixes a weak base with a strong acid? (Thanks for these excellent videos, by the way.)
• It is true when one mixes a weak base with a strong acid & also a weak acid with a strong base.
• At he says that at the half-equivalence point [A-]=[HA].

I thought that was true for the equivalence point, no ?
• No that’s correct.
At the half equivalence point half of HA has been neutralised, so half of it has been turned into A-
At the equivalence point all of HA has been neutralised and it’s all been turned into A-
• At , why is the concentration of the acid and base at the half equivalence point equal? I thought they were equal at the equivalence point.
(1 vote)
• Careful here because there are multiple acids involved in this titration. At the half-equivalence point the concentration of the base (the sorbate) and the conjugate acid to sorbate (sorbic acid) are equal. These are the concentrations being used for the Henderson–Hasselbalch equation Sal listed to calculate the pH.

At the equivalence point the concentration of the base (again the sorbate) is equal to the concentration of the acid being added to titrate the sorbate (otherwise known as the titrant). The titrant here is hydrochloric acid.

Hope that helps.
• What is A- in this rxn, and is Pottasium Sorbate a Weak Base?
(1 vote)
• When I am doing these types of questions, is it necessary to write out all of the equation for each question or do I just write the answer without all the math?
(1 vote)
• Is it the same with the equivalence point? because i thought that when ph=pka applies and not the half- equivalence point
(1 vote)
• Is what the same?
At the half equivalence point pH = pKa, not the equivalence point.
(1 vote)
• Wouldn't it be 4.8 because the least number of important numbers is 2. But you wrote 4.77. That's isn't in scientific notation.