The pH of a 0.6 M solution of a weak acid is 4.0. What percent of the acid has ionized?
A) 0.02%
B) 7%
C) 4%
D) 0.06%
E) 2%

Question

Quizplus · Accepted Answer

The correct answer is A because the percent of the acid that has ionized is 0.02%. This can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. In this case, the pH is 4.0 and the pKa is 4.0, so [A-]/[HA] = 1. This means that 50% of the acid has ionized. However, the concentration of the acid is 0.6 M, so the concentration of the conjugate base is 0.3 M. Therefore, the percent of the acid that has ionized is 0.3 M / 0.6 M * 100% = 50%.

The PH of a 0