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?

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.

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.