Calculate the [OH-] of a solution with a pH of 5.6.

Question

A. 5.6 x 10^-14 M

 B. 4.0 x 10^-9 M

 C. 3.1 x 10^-8 M

 D. 2.5 x 10^-6 M

Answer 1

pH = 5.6

pH = -log [H⁺]

[H⁺] = 1x10^-5.6 = 2.51x10^-6 M

[H⁺][OH^-] = 1x10^-14

[OH^-] = 1x10^-14 / 2.51x10^-6

[OH-] = 3.98x10^-9 M = 4x10^-9 M (answer B)

or you could do it this way:

pH = 5.6

pH = pOH = 14

pOH = 14 - 5.6 = 8.4

[OH-] = 1x10^-8.4

[OH-] = 3.98x10^-9 M = 4x10^-9