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Problem :

What is the ratio of base to acid when pH = pKa in a buffer? How about when pH = PKa + 1?

pH = pKa when the ratio of base to acid is 1 because log 1 = 0. When log (base/acid) = 1, then the ratio of base to acid is 10:1.

Problem :

Explain why the pKa of a buffer should be as close as possible to the desired pH.

The pKa should be quite close to the desired pH so that the ratio of base to acid in the Henderson-Hasselbalch equation will be close to 1. As the ratio of base to acid deviates from 1, the addition of acids and bases to the buffer will have a more profound effect on the pH.

Problem :

What is the pH of a buffered solution of 0.5 M ammonia and 0.5 M ammonium chloride when enough hydrochloric acid is dissolved to make it 0.15 M HCl? The pKb of ammonia is 4.75.

The pKa of ammonium ion is 9.25 since pKa = 14 - pKb. 0.15 M H+ reacts with 0.15 M ammonia to form 0.15 M more ammonium. Substituting the values of 0.65 M ammonium ion (acid) and 0.35 M remaining ammonia (base) into the Henderson-Hasselbalch equation gives a pH of 8.98.

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