A 35.0 mL sample of 0.150 M acetic acid solution is titrated with 0.150 M NaOH solution. Calculate the pH after the following volumes of base have been added.

Question

a. 15.5mL

Answer 1

The previous answer is incorrect. During the precipitation of a weak acid with a strong base the solution goes through a long buffer phase as the base converts the weak acid to its conjugate base.

There is 0.00293 mol CH₃COOH remaining and 0.002325 mol CH₃COO^- generated.

The Henderson Hasselbalch equation is a convenient method for obtaining pH for a buffer since we can use number of mol and converting to concentration is not needed.

pH = pKa + log(base/acid)

pH = 4.74 + log(0.002325/0.00293)

pH = 4.74 + (-0.10)

pH = 4.64

Checking this answer, it makes sense since there is a higher concentration of acid than base, so we expect the pH to be a little lower than the pKa