Ionization of water, and the dissociation constant of water, Kw

Water undergoes slight dissociation to produce hydronium (H₃O⁺) and hydroxyl (OH^-) ions. The dissociation can be expressed as autoprotolytic reaction of water in which one molecule of water reacts with another molecule of water to yield one molecule of hydronium ion and one molecule of hydroxyl ion as represented by the following reaction:

                                                         H₂O + H₂O ↔ H₃O⁺ +  OH^-

At equilibrium (rate of forward reaction =  rate of reverse reaction), the concentrations of all species (  H₂O, H₃O⁺, and OH^-) are constant. The dissociation constant of water (also known as autoprotolysis constant) can be expressed as the following:

                                                             Kw= [H₃O⁺] × [OH^-]

So Kw is equal to the hydronium ion concentration multiplied by the hydroxyl ion concentration. The value of Kw is equal to 1 × 10^-14 at 25 °C. Kw is affected by temperature and its value increases as temperature increase.

For pure water:
                                                               [H₃O⁺] = [OH^-] = X
                                                              Kw= [H₃O⁺] × [OH^-]
Thus,

                                                                      X² = 1 × 10^-14

So in pure water the hydronium ion and the hydroxyl ion concentrations are equal to 1 × 10^-7 moles/ liter (M) at 25 °C.

For a solution with 0.001 moles/ liter (M) HCl solution at 25 °C we can calculate the hydroxyl ion concentration as the following:
                                                          [H₃O⁺] = [HCl] = 0.001 M
                                                           1 × 10^-14= 0.001 × [OH^-]
                                                              [OH^-] = 1 × 10^-11  M

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