A student prepared a 0.10 M solution of acetic acid CH3COOH which has a
Ka of 1.75 x 10-5. What are the hydronium ion concentration and pH of the soln

We know that:

Ka = [H+][Ac-]/[HAc] (equilibrium equation);

Co = 0.10M = [HAc]+ [Ac-] (mass balance equation); and

[H+]= [Ac-] (charge-balance equation).

Therefore, [H+]^2/(0.10 - [H+]) = 1.75 x 10^-5.

Using the quadratic equation, [H+] = 1.31 x 10^-3 mol/L.

pH = -log10[H+] = 2.88.

Source: Chemical Principles, fourth edition, by Dickerson, Gray,
Darensbourg, and Darensbourg, The Benjamin/Cummings Publishing
Company, Inc., 1984, pages 163, and 179-181.

Sincerely,

Wonko