Question

[H^+] = 2.7 x 10^-3

Calculate for [OH^-]
[OH^-] = ? M

Answer 1

I do these by calculating pOH = -log(OH^-), then subtracting from 14.
pH + pOH = pKw = 14

Answer 2

The other way is [H^+]*[OH^-] = 10^-14