If a buffer has equal concentrations of a weak acid and its conjugate base, the pH is approximately equal to the acid’s pKa. Which statement best describes this?

If a buffer has equal concentrations of a weak acid and its conjugate base, the pH is approximately equal to the acid’s pKa. Which statement best describes this?

• A. The pH will be approximately equal to the pKa of the weak acid.
• B. The pH will be greater than 7 for any acid-base pair.
• C. The pH will be determined solely by the weak acid concentration, ignoring conjugate base.
• D. The pH will depend only on temperature.

Answer: (D)

The key idea is the Henderson–Hasselbalch relationship: a buffer’s pH depends on the ratio of conjugate base to weak acid. When [A−] equals [HA], the ratio is 1, so log([A−]/[HA]) is zero and pH equals pKa. That means the pH sits right at the acid’s pKa, reflecting the acid’s strength rather than the absolute amount of acid in the solution. It isn’t guaranteed that the pH will be above 7 for every acid–base pair, because that depends on the specific pKa of the acid. A buffer with a pKa below 7 will have a pH around that value, which is acidic, while a buffer with a higher pKa can be around neutral or basic. The pH is not set by the weak acid concentration alone; the conjugate base concentration matters as well, since the ratio drives the pH through the log term. Temperature also affects pH because pKa changes with temperature, but that’s not the sole determinant—the concentrations and their ratio are central.

The key idea is the Henderson–Hasselbalch relationship: a buffer’s pH depends on the ratio of conjugate base to weak acid. When [A−] equals [HA], the ratio is 1, so log([A−]/[HA]) is zero and pH equals pKa. That means the pH sits right at the acid’s pKa, reflecting the acid’s strength rather than the absolute amount of acid in the solution.

It isn’t guaranteed that the pH will be above 7 for every acid–base pair, because that depends on the specific pKa of the acid. A buffer with a pKa below 7 will have a pH around that value, which is acidic, while a buffer with a higher pKa can be around neutral or basic.

The pH is not set by the weak acid concentration alone; the conjugate base concentration matters as well, since the ratio drives the pH through the log term. Temperature also affects pH because pKa changes with temperature, but that’s not the sole determinant—the concentrations and their ratio are central.