# Determination of λ0M (limiting molar conductivity) for weak electrolytes

It is not possible to determine the λ

^{0}M 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,

λ

^{0}

_{CH3COOH}= λ

^{0}

_{H+}+ λ

^{0}

_{CH3COO-}………………………………… (Equation 1)

We can experimentally determine the λ

^{0}M of strong electrolytes such as HCl,CH3COONa and NaCl.

From Kohlraush’s law,

λ

^{0}

_{HCl}= λ

^{0}

_{H+}+ λ

^{0}

_{Cl-}……………………………………. (Equation 2)

λ

^{0}

_{CH3COONa}= λ

^{0}

_{Na+}+ λ

^{0}

_{CH3COO-}………………………………… (Equation 3)

λ

^{0}

_{NaCl}= λ

^{0}

_{Na+}+ λ

^{0}

_{Cl-}……………………………………. (Equation 4)

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

λ

^{0}

_{HCl}+ λ

^{0}

_{CH3COONa}- λ

^{0}

_{NaCl}= λ

^{0}

_{H+}+ λ

^{0}

_{Cl-}+ λ

^{0}

_{CH3COO-}+ λ

^{0}

_{Na+}- λ

^{0}

_{Na+}- λ

^{0}

_{Cl-}

= λ

^{0}

_{H+}+ λ

^{0}

_{CH3COO-}

= λ

^{0}

_{CH3COOH}

Thus by measuring the molar conductance values of NaCl, HCl and CH3COONa, one can easily determine the λ

^{0}

_{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}/ λ

^{0}

_{M}

Where λ

^{C}

_{M}is the molar conductivity at concentration ‘C’ and λ

^{0}

_{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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