8.3b Problems involving pH

The ionic product constant and pH equations

K_w = [H⁺][OH⁻] = 10⁻¹⁴

pH = − log[H⁺]

How do you get pH of a base?

Example: Find pH of 1M NaOH

First method:

1M NaOH (strong base), thus [OH-] = 1M

K_w = [H⁺][OH⁻] = 10⁻¹⁴

So  [H⁺] = 10⁻¹⁴ / [OH⁻] = 10⁻¹⁴ / 1M = 10⁻¹⁴

pH = − log[H⁺] = - log(10⁻¹⁴) = 14

Different way:

Likewise pOH = − log[OH-] = - log(1M) = 0

pH + pOH = 14

So pH = 14 - pOH = 14 - 0 = 14

Keep in mind inverse relationship between the two measures.