Question

What is the Keq for water at 25 C?

Answer 1

H2O dissociate into H3O+ and OH- itself. We call it as self-ionization. The term is given because any other solute there is need of solvent to be ionized. But water can itself ionizes into ions so it got the them ‘self-ionization.

We can show ionization reaction.

H2O + H2O --- > H3O + + OH-

According to the equilibrium expression we write ions to the numerator and H2O to the denominator.

K = [H3O+] [OH-] / [H2O][H2O]

The terms in the square bracket represent the concentration of them. As we know change in concentration in H2O is very negligible and there is very negligible change due to H2O in equilibrium expression. So we can write equilibrium expression without H2O concentration.

K = [H3O+][OH-]   

Since this is unique equilibrium developed from water so scientist named this equilibrium constant as K_w . Here subscript w stands for water.

K_w = [H3O+][OH-]

Its value at 25 deg C is 1.0 E-14

So we can also write it as

K_w = [H3O+][OH-] =1.0 E-14