Topic 12.1.3 Ionic product of water (Kw)

• Pure water has a slight electrical conductivity, so it must contain some ions. It self-ionises according to the following equation : H2O (l) ⇌ H+ (aq) + OH- (aq).
• H2O (l) is not included in the Kw expression, because it is a constant at a given temperature. However, H2O (g) is included.
• Water slightly dissociates to form hydroxide and hydrogen ions as an equilibrium with its own equilibrium constant (Kw).
• Kw = [H+][OH-]
• Kw is also known as the ionic product of water.
• The value of Kw at 25 degrees C (298K) is 1.0 x 10^-14. However, this value changes as temperature changes.
• Pure water has a pH of 7.00.
• A neutral solution is defined as one in which the hydrogen ion concentration is equal to the hydroxide ion concentration. Pure water is a neutral solution.
• pKw = -log Kw and Kw = 10 ^-pKw
• For a strong base, the concentration of OH- will be the same as the concentration of the base.