Chemical Equilibrium, Law Of Mass-action : Electrolytic Dissociation

If a salt, acid, or base is dissolved in water its molecules are broken up into ions. If the ionisation is incomplete, the degree of ionisation, α, will increase with dilution to a limiting value 1, when dissociation is complete. Let 1 mol of electrolyte be dissolved in a volume V. Let us consider the case of the ionisation of a weak acid:

HA(1 - α) <=> H^۰(α) + A'(α).

concentrations are then:

[HA] = (1 - α)/V; H^۰ = [A'] = α/V

If the law of mass-action applies to ionsation we shall have:

Ostwalds dilution law

This equation is known as Ostwald's dilution law (1888). It applies to weak acids, as may be seen from the values of K, the ionisation constant for acetic acid, and also to weak bases, such as ammonia: NH₄OH <=> NH₄^۰ + OH'. In the case of strong electrolytes, however, such as potassium chloride, it fails completely. Such electrolytes are now supposed to be completely ionised in dilute solutions.

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Chemical Equilibrium, Law Of Mass-action
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