Application of Kohlrausch's law

Determination of λ0M (limiting molar conductivity) for weak electrolytes

It is not possible to determine the λ⁰M for a weak electrolyte by the extrapolation of λ _M versus √c plot. But it can be calculated from Kohlrausch’s law. Consider the weak electrolyte CH3COOH. According to Kohlraush’s law,

λ⁰_CH3COOH = λ⁰_H+ + λ⁰_CH3COO- ………………………………… (Equation 1)

We can experimentally determine the λ⁰M of strong electrolytes such as HCl,CH3COONa and NaCl.
From Kohlraush’s law,

λ⁰_HCl = λ⁰_H+ + λ⁰_Cl- ……………………………………. (Equation 2)

λ⁰_CH3COONa = λ⁰_Na+ + λ⁰_CH3COO- ………………………………… (Equation 3)

λ⁰_NaCl = λ⁰_Na+ + λ⁰_Cl- ……………………………………. (Equation 4)

Substracting equation 4 from the sum of equations 2 and 3, we get,

λ⁰_HCl + λ⁰_CH3COONa - λ⁰_NaCl = λ⁰_H+ + λ⁰_Cl- + λ⁰_CH3COO- + λ⁰_Na+ - λ⁰_Na+ - λ⁰_Cl-

= λ⁰_H+ + λ⁰_CH3COO-

= λ⁰_CH3COOH

Thus by measuring the molar conductance values of NaCl, HCl and CH3COONa, one can easily determine the λ⁰_M (limiting molar conductivity)

Determination of degree of dissociation of weak electrolytes

The molar conductivity of a weak electrolyte depends up on the extent of dissociation. As the dilution increases, the degree of dissociation increases. The degree of dissociation of a weak electrolyte can be calculated by the relation :
Degree of dissociation α = λ^C_M / λ⁰_M
Where λ^C_M is the molar conductivity at concentration ‘C’ and λ⁰_M is the limiting molar conductance of the weak electrolyte. The limiting molar conductance can be calculated by Kohlrausch’s law.


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