Ionic Product of Water

Ionic Product of Water

Ionic Product of Water (K_w) is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself. That means water is behaving simultaneously as both an acid and a base.
H₂O ⇌ H⁺ + OH⁻
Equilibrium Constant-
K = [H⁺] [OH⁻]/[H₂O]
or, K . [H₂O] = [H⁺] [OH⁻]
or, K_w = [H⁺] [OH⁻]    (as the concentration of H₂O is constant)
The constant K_w, is called the dissociation constant or ionic product constant of water.
For pure water-
[H⁺] = [OH⁻] = 10⁻⁷M at 25^oC
so, pH = pOH = 7 at 25^oC
and K_w = 10⁻⁷ X 10⁻⁷ = 10⁻¹⁴
and, pK_w = 14
Number of water moles in 1 liter of water = 1000/18 = 55.5 moles
number of water molecules in 1 liter water = 55.5 N_A
Concentration of H⁺ or OH⁻ ions = 10⁻⁷mol/liter

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