Question

What is the pH of a 4.1×10-8 molar aqueous perchloric acid solution? (Hint: The H3O+ due to the water ionization is not negligible here.)

Answer 1

Given the concentration of perchloric acid (HClO4), C = 4.1x10^-8 M

Since perchloric acid (HClO4) is a strong acid it is completely dissociated in water.

Hence concentration of H₃O⁺ formed due to dissociation of HClO4, C1 = 4.1x10^-8 M

Also the concentration of H₃O⁺ formed due to auto-ionization of water, C2 = 10^-7 M

Hence total concentration of H₃O⁺, [H₃O⁺ ] = C1+C2 = 4.1x10^-8 M + 10^-7 M = 1.41x10^-7 M

Hence pH = - log[H₃O⁺ ] = - log(1.41x10^-7 M) = 6.85 (answer)