A buffer composed of a weak acid with pKa 4.76 and its conjugate base is prepared at total concentration 0.1 M. If the acid and base are present in equal amounts, what is the expected pH?

A buffer composed of a weak acid with pKa 4.76 and its conjugate base is prepared at total concentration 0.1 M. If the acid and base are present in equal amounts, what is the expected pH?

• A. 4.56
• B. 4.00
• C. 4.76
• D. 4.90

Answer: (C)

The essential idea here is how a buffer's pH is predicted when a weak acid and its conjugate base are present. Use Henderson–Hasselbalch: pH = pKa + log([A-]/[HA]). If the acid and its conjugate base are in equal amounts, the ratio [A-]/[HA] is 1, and log(1) = 0. Therefore, pH equals pKa. The given pKa is 4.76, so the expected pH is 4.76. The total concentration (0.1 M) doesn’t change this result in the ideal Henderson–Hasselbalch framework.

The essential idea here is how a buffer's pH is predicted when a weak acid and its conjugate base are present. Use Henderson–Hasselbalch: pH = pKa + log([A-]/[HA]). If the acid and its conjugate base are in equal amounts, the ratio [A-]/[HA] is 1, and log(1) = 0. Therefore, pH equals pKa. The given pKa is 4.76, so the expected pH is 4.76. The total concentration (0.1 M) doesn’t change this result in the ideal Henderson–Hasselbalch framework.