Asolution has [h3o+] = 4.2×10−5 m . use the ion product constant of water kw=[h3o+][oh−] to find the [oh−] of the solution.

Answer is: concentration of hydroxide ions is 2.38·10⁻¹⁰ M.
[H₃O⁺] = 4.2·10⁻⁵ M; equilibrium concentration of hydroxide ions.
The Kw (the ionic product for water) at 25°C is 1·10⁻¹⁴ mol²/dm⁶ or 1·10⁻¹⁴ M².
Kw = [H₃O⁺] · [OH⁻].
[OH⁻] = Kw ÷ [H₃O⁺].
[OH⁻] = 1·10⁻¹⁴ M² ÷ 4.2·10⁻⁵ M.
[OH⁻] = 2.38·10⁻¹⁰ M.

Gallium melts near room temperature and has one of the largest  liquid  ranges of any metal, so it has found use in high temperature thermometers.