The Ionic Product of Water Kw

Pure water ionises into H⁺(aq) and OH^-(aq) ions to a very tiny extent.H₂O(l) ⇌ H⁺(aq) + OH^-(aq)The equilibrium lies to the far left and there are very few ions. The dissociation is endothermic and requires energy. The expression for the acid dissociation constant isSince both [H₂O(l)] and K_a are constant, this can be combined into one constant called K_w.K_w = [OH^-][H⁺]At 25°C K_w = 1.0 x 10^-14 mol²dm^-6. This is the neutral point where [H⁺] = [OH^-] for pure water so the [H⁺] at 25°C is 1.0 x 10^-7