Ionization Constant for acid

Question

A 0.085 M solution a monoprotic acid HA has a percent ionization of 1.5%. Determine the acid ionization constant K_a for the acid

Answer 1

HA <==> H⁺ + A^-

Ka = [H+][A-]/[HA]

% ionization = [H+]/[HA] = 1.5% = [H+]/0.085

0.015 = [H+]/0.085

Solving for [H+]:

[H+] = 0.001275 This is also the [A-]

Ka = (0.001275)(0.001275)/0.085

Ka = 1.9x10^-5