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

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.