The molecular weighs of water is 18.015 and 1L of pure water at 25^{0}C weighs about 1000g. Therefore, the molar concentration of water is 1000/18.015 = 55.509 mol L^{-1}. One molecule of water in every 10^{7} 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.

H_{2}O = H^{+} + OH^{-}

The dissociation constant, K_{a}, of water at 25^{0}C is 1.8 x 10^{-16} .

Since concentration of H_{2}O is 55.509 M and [H^{+}] is negligible,

so, [H_{2}O] - [H^{+}] → H_{2}O

[H^{+}] x [OH^{-}] = 1.8 x 10^{-16} x [H_{2}O]

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

[H^{+}] x [OH^{-}] = 9.99162 x 10^{-15}

Ion product of water, K_{w}, is given by,

K_{w} = [H^{+}] x [OH^{-}] (2)

K_{w }= 9.99162 x 10^{-15} (3)

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

K_{w} = [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 CO_{2} from atmosphere.

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

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