Kohlraush’s Law and its Applications

Kohlrausch's law states that each ion contributes a definite amount to an electrolyte's molar conductance at infinite dilution, Λ°_m, regardless of its counter ion.

Here, Λ°_m is the sum of these individual ionic contributions, known as molar ionic conductances at infinite dilution.

This law facilitates the calculation of the transport number of cations and anions, as the molar ionic conductance of each ion is directly proportional to the mobility of the ion at infinite dilution.

Additionally, Kohlrausch's law also enables the indirect calculation of Λ°_m for weak electrolytes, which assists in determining their degree of dissociation, expressed as the ratio of molar conductance, Λ_m to Λ°_m.

For example, the Λ°_m of acetic acid can be obtained by adding the experimentally determined Λ°_m values of HCl and sodium acetate, then subtracting the Λ°_m of NaCl.

Similarly, the Λ°_m of a sparingly soluble salt like AgCl can be calculated by adding the Λ°_m values of strong electrolytes such as NH₄Cl and AgNO₃, and then subtracting the Λ°_m of NH₄NO₃.