# 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,

λ0CH3COOH = λ0H+ + λ0CH3COO- ………………………………… (Equation 1)

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

λ0HCl = λ0H+ + λ0Cl- ……………………………………. (Equation 2)

λ0CH3COONa = λ0Na+ + λ0CH3COO- ………………………………… (Equation 3)

λ0NaCl = λ0Na+ + λ0Cl- ……………………………………. (Equation 4)

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

λ0HCl + λ0CH3COONa - λ0NaCl = λ0H+ + λ0Cl- + λ0CH3COO- + λ0Na+ - λ0Na+ - λ0Cl-

= λ0H+ + λ0CH3COO-

= λ0CH3COOH

Thus by measuring the molar conductance values of NaCl, HCl and CH3COONa, one can easily determine the λ0M (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 = λCM / λ0M
Where λCM is the molar conductivity at concentration ‘C’ and λ0M is the limiting molar conductance of the weak electrolyte. The limiting molar conductance can be calculated by Kohlrausch’s law.

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