Acids and Bases: Buffers

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Topics Problems and Solutions

Problem :

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

pH = pK_a 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 pK_a of a buffer should be as close as possible to the desired pH.

The pK_a 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 pK_b of ammonia is 4.75.

The pK_a of ammonium ion is 9.25 since pK_a = 14 - pK_b. 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.