HZ is a weak acid. An aqueous solution of HZ is prepared by dissolving 0.020 mol of HZ in sufficient water to yield 1.0 L of solution. The pH of the solution was 4.93 at 25.0°C. The K_a of HZ is __________.

Question

Quizplus · Accepted Answer

- We need to find K_a of HZ, which is the acid dissociation constant. - Knowing the pH of the solution, we can calculate the concentration of H+ ions using the expression pH = -log[H+]. - pH = 4.93 implies [H+] = 10^-4.93 M. - Since HZ is a weak acid, we can assume that most of the H+ ions come from the dissociation of HZ: HZ + H2O ⇌ H3O+ + Z-. - The equilibrium constant expression for this reaction is Ka = [H3O+][Z-]/[HZ]. (Note that [H2O] is not included because it is a pure liquid and does not affect the equilibrium.) - At equilibrium, the concentration of HZ that has dissociated is equal to [H3O+]. Therefore, we can simplify the expression to Ka = [H3O+]^2/[HZ]. - Substituting the values we know: Ka = (10^-4.93)^2/0.020 M = 6.9 × 10^-10 M. Therefore, the correct answer is B, 6.9 × 10^-10.

HZ Is a Weak Acid