For a buffer containing equal concentrations of a weak acid and its conjugate base, the pH is approximately equal to...

For a buffer containing equal concentrations of a weak acid and its conjugate base, the pH is approximately equal to...

• A. pH ≈ 4.74
• B. pH ≈ 7.00
• C. pH ≈ pKa
• D. pH ≈ 2×pKa

Answer: (C)

When a buffer has equal amounts of a weak acid and its conjugate base, the ratio [A-]/[HA] is 1. The Henderson–Hasselbalch equation says pH = pKa + log([A-]/[HA]); with log(1) = 0, this gives pH = pKa. So the pH of such a buffer is determined by the acid’s pKa, not by a fixed number. The correct takeaway is that the pH is approximately equal to the acid’s pKa. A specific number like 4.74 would only apply if the acid in the buffer has pKa 4.74 (for example, acetic acid). The idea of pH being 2×pKa isn’t correct because the ratio is 1, not 1/2 or 2. Saying pH ≈ 7 would be a special case only if the acid’s pKa happens to be around 7.

When a buffer has equal amounts of a weak acid and its conjugate base, the ratio [A-]/[HA] is 1. The Henderson–Hasselbalch equation says pH = pKa + log([A-]/[HA]); with log(1) = 0, this gives pH = pKa. So the pH of such a buffer is determined by the acid’s pKa, not by a fixed number. The correct takeaway is that the pH is approximately equal to the acid’s pKa.

A specific number like 4.74 would only apply if the acid in the buffer has pKa 4.74 (for example, acetic acid). The idea of pH being 2×pKa isn’t correct because the ratio is 1, not 1/2 or 2. Saying pH ≈ 7 would be a special case only if the acid’s pKa happens to be around 7.