What is the hydroxide ion concentration in an aqueous solution with a hydrogen ion concentration of 2.70E–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)