# Applications of Kohlrausch's Law

Some typical applications of the Kohlrausch's law are described below,

(i) **Determination of _{} for weak electrolytes** : The molar conductivity of a weak electrolyte at infinite dilution

_{}cannot be determined by extrapolation method. However,

_{}values for weak electrolytes can be determined by using the Kohlrausch's equation.

_{}

(ii) **Determination of the degree of ionisation of a weak electrolyte**: The Kohlrausch's law can be used for determining the degree of ionisation of a weak electrolyte at any concentration. If _{} is the molar conductivity of a weak electrolyte at any concentration C and, _{} is the molar conductivity of a electrolyte at infinite dilution. Then, the degree of ionisation is given by, _{}

Thus, knowing the value of _{}, and _{} (From the Kohlrausch's equation), the degree of ionisation at any concentration _{} can be determined.

(iii) **Determination of the ionisation constant of a weak electrolyte **: Weak electrolytes in aqueous solutions ionise to a very small extent. The extent of ionisation is described in terms of the degree of ionisation _{}In solution, the ions are in dynamic equilibrium with the unionised molecules. Such an equilibrium can be described by a constant called **ionisation constant**. For example, for a weak electrolyte AB, the ionisation equilibrium is, _{} ⇌ _{}; If C is the initial concentration of the electrolyte AB in solution, then the equilibrium concentrations of various species in the solution are, _{} _{} and _{}

Then, the ionisation constant of AB is given by, _{}

We know, that at any concentration C, the degree of ionisation _{} is given by, _{}

Then, _{}; Thus, knowing _{} and _{} at any concentration, the ionisation constant (K) of the electrolyte can be determined.

(iv) **Determination of the solubility of a sparingly soluble salt** : The solubility of a sparingly soluble salt in a solvent is quite low. Even a saturated solution of such a salt is so dilute that it can be assumed to be at infinite dilution. Then, the molar conductivity of a sparingly soluble salt at infinite dilution _{} can be obtained from the relationship,

_{} ........(i)

The conductivity of the saturated solution of the sparingly soluble salt is measured. From this, the conductivity of the salt _{}can be obtained by using the relationship, _{}, where, _{} is the conductivity of the water used in the preparation of the saturated solution of the salt.

_{} ........(ii)

From equation (i) and (ii) ;

_{}, _{} is the molar concentration of the sparingly soluble salt in its saturated solution. Thus, _{} is equal to the solubility of the sparingly soluble salt in the mole per litre units. The solubility of the salt in gram per litre units can be obtained by multiplying _{} with the molar mass of the salt.