The Chemistry Of Biology Practice Test

Using the Henderson-Hasselbalch equation, what is the pH for a buffer with pKa 4.76 and [A-]/[HA] = 1?

5.76 3.76 4.76 4.00

Correct Answer:

4.76

Explanation:

The idea being tested is how the Henderson-Hasselbalch equation links pH to the pKa and the ratio of conjugate base to acid. The equation states pH = pKa + log([A-]/[HA]). If the ratio is 1, meaning equal amounts of A- and HA, log(1) = 0, so pH equals pKa. Here the pKa is 4.76, and with [A-]/[HA] = 1, the pH is 4.76. This shows that a buffer has a pH equal to its pKa when the concentrations of the acid and its conjugate base are equal, which is also the point where the buffer resists changes in pH most effectively. If there were more base than acid, pH would be higher; if more acid than base, pH would be lower.

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