An acetate buffer contains equal volumes of 0.35 M HC2H3O2 (pKa = 4.74) and 0.55 M NaC2H3O2. What is the pH of the buffer?

An acetate buffer contains equal volumes of 0.35 M HC2H3O2 (pKa = 4.74) and 0.55 M NaC2H3O2. What is the pH of the buffer?

• A. 4.74
• B. 4.94
• C. 4.54
• D. 7.00

Answer: (B)

The pH of an acetate buffer is given by the Henderson–Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the conjugate base (acetate) and [HA] is the weak acid (acetic acid). With equal volumes, the concentrations you use are directly the given molarities: [A-] = 0.55 M and [HA] = 0.35 M. The ratio is 0.55/0.35 ≈ 1.571, and log10(1.571) ≈ 0.196. Therefore pH ≈ 4.74 + 0.196 ≈ 4.94. Because the base form is in excess, the pH ends up slightly above the pKa, as expected for a buffer where [A-] > [HA]. If the volumes weren’t equal, the ratio would change and the pH would shift accordingly, even though the buffer’s capacity would also change with total concentration.

The pH of an acetate buffer is given by the Henderson–Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the conjugate base (acetate) and [HA] is the weak acid (acetic acid). With equal volumes, the concentrations you use are directly the given molarities: [A-] = 0.55 M and [HA] = 0.35 M. The ratio is 0.55/0.35 ≈ 1.571, and log10(1.571) ≈ 0.196. Therefore pH ≈ 4.74 + 0.196 ≈ 4.94. Because the base form is in excess, the pH ends up slightly above the pKa, as expected for a buffer where [A-] > [HA]. If the volumes weren’t equal, the ratio would change and the pH would shift accordingly, even though the buffer’s capacity would also change with total concentration.