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.
