The pH of a 0.14 M solution of a weak monoprotic acid, HA, is 2.89. Calculate K_a for this acid.

Question

Quizplus · Accepted Answer

The pH of the solution is 2.89, which means the concentration of H+ ions is 10^-2.89. For a weak acid, HA ⇌ H+ + A-, the concentration of H+ ions is equal to the concentration of A- ions at equilibrium. The acid dissociation constant, Ka, can be calculated using the formula Ka = [H+][A-]/[HA], where [HA] is the initial concentration of the acid minus the concentration of H+ ions. Since the concentration of H+ ions is much smaller than the initial concentration of HA, we can approximate [HA] ≈ 0.14 M. Therefore, Ka ≈ [H+]^2 / [HA] = (10^-2.89)^2 / 0.14 = 1.29 x 10^-6, which is closest to 1.7 × 10^-6, option A.

The PH of a 0