Question

Write a note on ionic product (K_w) of water

Answer 1

Pure water ionises to a very small extent.The ionization equilibrium of water is represented as,

\[\ce{H2O(l) + H2O(l) <=> H3O^⊕(aq) + OH^Θ(aq)}\]

The equilibrium constant (K) for the ionization of water is given by

`K = ([H_3O^⊕][OH^Θ])/[H2O]^2       ...(1)`      

`K[H_2O]^2=[H_3O^⊕][OH^Θ]         ...(2)`

A majority of H₂O molecules are undissociated, consequently concentration of water [H₂O] can be treated as constant. Then [H₂O]² = K'.

Substituting this in Equation (2) we get,

K x K' = [H₃O^⊕][OH^Θ]       ...(3)

K_w = [H₃O^⊕][OH^Θ]  

where K_w = KK' is called ionic product of water. The product of molar concentrations of hydronium (or hydrogen) ions and hydroxyl ions at equilibrium in pure water at the given temperature is called ionic product of water.

In pure water H3O^⊕ ion concentration always equals the concentration of OH^Θ ion. Thus at 298K this concentration is found to be 1.0 × 10^-7 mol/L.

K_w = (1.0 × 10^-7) (1.0 × 10^-7)

K_w = 1.0 × 10^-14