# SSC 601: Advanced Soil Chemistry

## Ion product of water

The molecular weighs of water is 18.015 and 1L of pure water at 250C weighs about 1000g. Therefore, the molar concentration of water is 1000/18.015 = 55.509 mol L-1. One molecule of water in every 107 molecule dissociate into H+ and OH- ions. However, this slight dissociation of water is very essential for many of chemical processes and reactions in water.

H2O = H+ + OH-

The dissociation constant, Ka, of water at 250C is 1.8 x 10-16 .

Since concentration of H2O is 55.509 M and [H+] is negligible,

so, [H2O] - [H+] → H2O

[H+] x [OH-] = 1.8 x 10-16 x [H2O]

[H+] x [OH-] = 1.8 x 10-16 x 55.509

[H+] x [OH-] = 9.99162 x 10-15

Ion product of water, Kw, is given by,

Kw = [H+] x [OH-]                                  (2)

Kw = 9.99162 x 10-15                              (3)

Since, concentration of H+ and OH- in water are equal, therefore, equation (3) can be written as,

Kw = [H+] x [OH-] = [H+]2                        (4)

From equation (3) and (4), we can get,

[H+]2 = 9.99162 x 10-15 = 10-14                     (5)

Taking negative log on both the sides, equation (5), becomes,

2pH = 14.0

pH = 7.0

The pH of pure water is 7.0 under control conditions, but under ordinary laboratory conditions, pH of distilled water is acidic (pH ≈ 5.0) because of removal of basic cations during distillation and absorption of CO2 from atmosphere.

Remember that a meq of any cation is that amount of cation required to replace 1 meq of another cation.