Question

State Kohlrausch law. How is it useful to determine the molar conductivity of weak electrolytes at infinite dilution?

Answer 1

Kohlrausch’s law: It is defined as at infinite dilution the limiting molar conductivity of an electrolyte is equal to the sum of the limiting molar conductivities of its constituent ions.

Determination of the molar conductivity of weak electrolytes at infinite dilution.

It is impossible to determine the molar conductance at infinite dilution for weak electrolytes experimentally. However, the same can be calculated using Kohlraushs Law. For example, the molar conductance of CH₃COOH can be calculated using the experimentally determined molar conductivities of strong electrolytes HCI, NaCI and CH₃COONa.

$$\mathrm{\Lambda }{\phantom{A}}_{{\mathrm{CH}}_3\mathrm{COONa}}^{\circ }=\mathrm{\lambda }{\phantom{A}}_{{\mathrm{Na}}^{+}}^{\circ }+\mathrm{\lambda }{\phantom{A}}_{{\mathrm{CH}}_3\mathrm{COONa}}^{\circ }$$ .....(1)

$$\mathrm{\Lambda }{\phantom{A}}_{\mathrm{HCl}}^{\circ }=\mathrm{\lambda }{\phantom{A}}_{{\mathrm{H}}^{+}}^{\circ }+\mathrm{\lambda }{\phantom{A}}_{{\mathrm{Cl}}^{-}}^{\circ }$$ ......(2)

$$\mathrm{\Lambda }{\phantom{A}}_{\mathrm{NaCl}}^{\circ }=\mathrm{\lambda }{\phantom{A}}_{{\mathrm{Na}}^{+}}^{\circ }+\mathrm{\lambda }{\phantom{A}}_{{\mathrm{Cl}}^{-}}^{\circ }$$ .........(3)

Equation (1) + Equation (2) – Equation (3) gives,

$$(\mathrm{\Lambda }{\phantom{A}}_{{\mathrm{CH}}_3\mathrm{COONa}}^{\circ })+(\mathrm{\Lambda }{\phantom{A}}_{\mathrm{HCl}}^{\circ })-(\mathrm{\Lambda }{\phantom{A}}_{\mathrm{NaCl}}^{\circ })=\mathrm{\lambda }{\phantom{A}}_{{\mathrm{H}}^{+}}^{\circ }+\mathrm{\lambda }{\phantom{A}}_{{\mathrm{CH}}_3\mathrm{COONa}}^{\circ }=\mathrm{\Lambda }{\phantom{A}}_{{\mathrm{CH}}_3\mathrm{COONa}}^{\circ }$$