Question

What is the hydroxide ion concentration in an aqueous solution with a hydrogen ion concentration of
concentration
2.70 x 10-2 M?

Answer 1

[H+][OH-] = 1x10-14 = Kw (Memorize this relationship)
(2.70x10-2)[OH-] = 1x10-14
[OH-] = 1x10-14/2.70x10-2
[OH-] = 3.70x10-13 M (assuming you ignore the autoionization of H2O)