Answer is: the
hydrogen ion concentration is 2,5·10⁻⁷ M.
[OH⁻] = 4,0·10⁻⁸ mol/L, equilibrium concentration of hydroxide
anion.
[H⁺]
is the concentration of hydrogen ions.
[OH⁻] ·
[H⁺] = 10⁻¹⁴ mol²/L², ionic product of water on room temperature.
[H⁺] =
10⁻¹⁴ mol²/L² ÷ 4·10⁻⁸ mol/L.
[H⁺] = 2,5·10⁻⁷ mol/L = 0,00000025 mol/L.
