10^-14 / [H3O+]
Explanation:
Hydronium (H3O+) and hydroxide ions (OH-) are both present in pure water and in all aqueous solutions.
Their respective concentrations in water are 10^-7 M each and are inversely proportional to each other as given by the ion product of water, Kw.
Kw = [H3O+][OH−]
Where Kw = 1.0 * 10^-14,
[H3O+] = concentration of hydronium ions
[OH-] = concentration of hydroxide ions
Therefore, [OH-] = Kw / [H3O+]
[OH-] = 10^-14/[H3O+]
