Acids and Bases: Buffers

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.

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.

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.