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pH scale

Acidity and basicity of a solution is measured by pH. pH is defines as the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions [H3O+].

pH = -log

H2OH+ + OH- 

The dissociation constant: .

Here  [H2O] is constant, So,
K [H2O] = [H+] [OH-] = Kw

Kw is an ionic product of water and its value is 1x10-14 at 25°C.

+] [OH-] = 1x10-14

Pure water dissociate completely and has equal concentration. Thus [H+] = [OH-]

+] [H+] = 1x10-14
 [H+] = 10-7
pH = 7

Thus, pure water is a neutral solution having pH equal to 7.0.

Acidic soution pH < 7
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