A solution has a hydroxide ion concentration of 1 * 10-5 M. What is the hydrogen ion concentration of the solution?

(a) 1 * 10-1 M

(b) 1 * 10-5 M

(c) 1 * 10-9 M

(d) 1 * 10-14 M

The correct answer is (c) 1 * 10⁻⁹ M

Explanation:

Hydroxide ion = OH⁻

Hydrogen ion = H⁺

The autoionization equilibrium of water at 25ºC has a water constant Kw which is expressed as follows:

Kw = [H⁺] x [OH⁻] = 1 x 10⁻¹⁴

If we know the concentration of hydroxide ion ([OH⁻]) we can calculate the hydrogen ion concentration ([H⁺]) as follows:

[H⁺] = Kw/[OH⁻] = (1 x 10⁻¹⁴) / 1 x 10⁻⁵ M = 1 x 10⁻⁹ M