A 0.250 M aqueous solution of p -toluic acid has a pH of 2.49. What is K _a for this weak acid?

Question

Quizplus · Accepted Answer

The pH of the solution is 2.49, which means the concentration of H+ ions is 10^-2.49. For a weak acid HA ↔ H+ + A-, the ionization can be represented as Ka = [H+][A-]/[HA]. Given the initial concentration of the acid is 0.250 M and assuming x is the concentration of H+ ions that dissociate, [HA] at equilibrium is approximately 0.250 M (since x is small compared to 0.250 M). The concentration of H+ and A- at equilibrium is x, which is 10^-2.49. Therefore, Ka = (10^-2.49)^2 / 0.250 = 10^-4.98 / 0.250 ≈ 4.2×10^-5, matching choice A.

A 0250 M Aqueous Solution of P -Toluic Acid Has a a PH