The pKa for acetic acid is 4.76. If the concentration of salt is 5 mmol/L and that of acetic acid is 10 mmol/L, what is the expected pH?

• A. 4.46
• B. 5.06
• C. 104
• D. 56

Answer: (A)

The pH of this solution is determined by the Henderson–Hasselbalch relationship for a weak acid and its conjugate base: pH = pKa + log([A-]/[HA]). Here, the salt provides the conjugate base A- at 5 mmol/L, and the acetic acid provides the weak acid HA at 10 mmol/L. The ratio [A-]/[HA] is 5/10 = 0.5. So pH = 4.76 + log(0.5) = 4.76 + (-0.301) ≈ 4.46. Since there is more acid than base, the pH is below the pKa, as expected. The correct pH is about 4.46.