Acid Base PH, POH

Question

What is the pH of a solution for which [OH^−] equals 7.87 × 10^−10 M?

Answer 1

We can solve this one of two ways: Find the pOH then subtract from 14.00 is one way.

pOH = - log [OH^-] = - log [7.87 x 10^-10] = 9.10 14.00 - 9.10 = 4.90

Other way:

K_w = [H⁺][OH^-]

[H⁺] = K_w/[OH^-] = 1.00 x 10^-14/7.87 x 10^-10 = 1.27 x 10^-5

pH = - log [H⁺] = 4.90