Question

Neglecting activity coefficients, calculate the pH of 4.5 ✕ 10−8 M HClO4. (Assume Kw = 1.01 ✕ 10−14. Enter an unrounded value.)

6.83863199777 Correct: Your answer is correct.

What fraction of the total [H + ] is derived from dissociation of water? (Enter an unrounded value.)

0.69309487042 Incorrect: Your answer is incorrect.

0.68965517241 Incorrect: Your answer is incorrect.

As you can see, the 1st part is correct, I am having trouble getting the second part of the question. Please explain in detail.  

Answer 1

[H+]total = H+ from water + H+ from HClO4

              = 1.0 x10^-7 + 4.5 x 10^-8

               = 1.45 x10^-7 M

pH = -log [H+] = 6.839

pH = 6.839

fraction of H+ = H+ from HClO4 / total H+

                        = 4.5 x 10^-8 / 1.45 x10^-7

                        = 0.310