Question

If the Ka of a monoprotic weak acid is 3.4 × 10-6, what is the pH of a 0.34 M solution of this acid?

Answer 1

lets mono protic acid = HA

construct ICE table

     HA (aq) + H2O (l) <----> H3O+ (aq) + A- (aq)

I       0.34                             0                   0

C     -x                                 +x                 +x

E = 0.34-x                            +x                   +x

Ka = [H3O+] [A-] / [HA]

3.4 x 10^-6 = [x] [x] /[0.34 -x]

x² + x 3.4 * 10^-6 - 1.156 x 10^-6 = 0

solve the quadratic equation

x = 0.001073 M = [H3O+]

pH = -log[H3O+] = -log[0.001073M]

pH = 2.97