The pH of a 0.100 M solution of an aqueous weak acid (HA) is 3.20. What is K_a for the Weak acid? A) 4.0 × 10^-6 B) 6.3 × 10^-4 C) 7.2 × 10^-5 D) 3.2 E) none of these

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The pH of the solution is 3.20, which means the concentration of H+ ions is 10^-3.20 M. For a weak acid HA dissociating into H+ and A-, the equilibrium concentration of H+ (and A-) will be equal to the square root of Ka * [HA]initial, assuming x is small compared to the initial concentration. Since [H+] = 10^-3.20, Ka = [H+]^2 / [HA]initial = (10^-3.20)^2 / 0.100 = 10^-6.4 / 0.100 = 4.0 × 10^-6, matching choice A.

The PH of a 0