Question

What is the concentration of OH^– in a solution with a H^+ concentration of 1.3×10^-4 M?

(a) 7.7×10^-10  M

(b) 7.7×10^-9  M

(c) 7.7×10^-11  M

(d) 7.7×10^-12  M

This question was posed to me in an interview.

Question is taken from Ionization of Water, Weak acids and Weak Bases topic in portion Water Basics of Biochemistry

Answer 1

The correct answer is (c) 7.7×10^-11  M

To explain I would say: Kw = [H^+][OH^–]

Solving for [OH^–]  gives

     [OH^–]=(Kw)/([H^+]) = (1.0 X 10^-14)/(1.3 X 10^-4) M

                           = 7.7 × 10^-11  M.