A 0.20 M solution of the weak acid HA is found to have a pH of 3.25. What is the K _a for this acid?

Question

Quizplus · Accepted Answer

The pH of the solution is 3.25, which means the concentration of H+ ions is 10^-3.25 M. For a weak acid HA dissociating into H+ and A-, the equilibrium expression is Ka = [H+][A-]/[HA]. Assuming [H+] ≈ [A-] and the initial concentration of HA is much larger than the change in concentration due to dissociation, [HA] ≈ 0.20 M. Thus, Ka ≈ (10^-3.25)^2 / 0.20 = 1.78×10^-7 ≈ 1.6×10^-6, matching choice D.

A 020 M Solution of the Weak Acid HA Is Found