Which expression defines the ion-product constant for water, Kw?

• A. Kw = [H2O][OH-]
• B. Kw = [H3O+][OH-]
• C. Kw = [H+][OH-]
• D. Kw = [H3O+][H2O]

Answer: (B)

Water partially ionizes by forming hydronium and hydroxide: H2O ⇌ H3O+ + OH−. The ion-product constant Kw at a given temperature is defined from the activities of these ions. In dilute aqueous solutions, activities are well approximated by concentrations, and the solvent water is effectively constant, so Kw becomes the product of the two ion concentrations: [H3O+][OH−]. This is the quantity that remains essentially the same regardless of how much water is present, which is why the expression uses hydronium and hydroxide rather than water. Using [H3O+][H2O] would make Kw depend on how much water there is, which isn’t the case, and while [H+][OH−] is commonly used in a loose sense, the precise form uses hydronium to reflect the actual species in solution. At 25°C, Kw is about 1×10−14.