The pH of a 0025 M solution of a certain weak acid is 414 Wh

The pH of a 0.025 M solution of a certain weak acid is 4.14. What is the percent ionization of this acid?

Solution

The weak acid, HA, dissolves into protons, H+, and its conjugate base, A-. The HA is not completely ionized, however, and only some H+ appears in solution, contributing to the pH. You are given pH = 4.14, so you can determine the H+ concentration:
pH = -log[H+]
[H+] = 10^-pH
[H+] = 10^-4.14 = 1x10^-4.14 M

Since the original solution of the weak acid is 0.025M, and it is ionizing to give 1x10^-4.14 M H+, we can determine the percentage:
100(1x10^-4.14 / 0.025) = 0.289 %