7.11.1 The Ionization Constant of Water and its Ionic Product

Some substances like water are unique in their ability of acting both as an acid and a base. We have seen this in case of water in section 7.10.2.

In presence of an acid, HA it accepts a proton and acts as the base while in the presence of a base, B it acts as an acid by donating a proton. In pure water, one H2O molecule donates proton and acts as an acid and another water molecules accepts a proton and acts as a base at the same time.

The following equilibrium exists:

H2O(I) +  H2O(l)    ⇔     H3O+(aq)         +     OH-(aq)

Acid         base               conjugate acid        conjugate base

 

The dissociation constant is represented by,

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

The concentration of water is omitted from the denominator as water is a pure liquid and its concentration remains constant.

[H2O] is incorporated within the equilibrium constant to give a new constant, Kw, which is called the ionic product of water.

Kw = [H+] [OH-]

The concentration of H+ has been found out experimentally as 1.0 × 10–7 M at 298 K. And, as dissociation of water produces equal number of H+ and OH ions, the concentration of hydroxyl ions,

[OH] = [H+] = 1.0 × 10–7 M. Thus, the value of Kw at 298K,

Kw = [H3O+][OH-] = (1 × 10-7)2 = 1 × 10-14 M2

The value of Kw  is temperature dependent as it is an equilibrium constant.

The density of pure water is 1000 g / L and its molar mass is 18.0 g /mol. From this the molarity of pure water can be given as,

[H2O] = (1000 g /L)(1 mol/18.0 g) = 55.55 M.

Therefore, the ratio of dissociated water to that of undissociated water can be given as:

10-7/ (55.55) = 1.8 × 10-9or ~ 2 in 10-9 (thus, equilibrium lies mainly towards undissociated water)

We can distinguish acidic, neutral and basic aqueous solutions by the relative values of the H3O+ and OH concentrations:

Acidic: [H3O+] > [OH-]

Neutral: [H3O+] = [OH-]

Basic: [H3O+] < [OH-]

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