Kohlrausch's law

Kohlrausch's Law

Kohlrausch's Law is a key concept in electrochemistry that describes how electrolytes behave when they are infinitely diluted. It is particularly useful for determining the molar conductance of weak electrolytes, which cannot be measured directly due to incomplete dissociation.

Statement of Kohlrausch's Law

According to Kohlrausch’s Law, the molar conductivity of an electrolyte at infinite dilution is the combined contribution of the individual conductivities of its cations and anions.

Λ_m^∞ = λ₊^∞ + λ_–^∞

Where:
Λ_m^∞ = Molar conductance at infinite dilution
λ₊^∞ = Limiting molar conductance of the cation
λ_–^∞ = Limiting molar conductance of the anion

Applications of Kohlrausch’s Law

• Helps calculate the molar conductance of weak electrolytes at infinite dilution.
• Used to determine the degree of dissociation (α) and dissociation constant (K_a) of weak electrolytes.
• Helps determine whether an electrolyte is strong or weak.

Example

In the case of acetic acid (CH₃COOH), a weak electrolyte:

Λ_m^∞(CH₃COOH) = Λ_m^∞(CH₃COONa) + Λ_m^∞(HCl) – Λ_m^∞(NaCl)

Kohlrausch's Law provides a reliable method to estimate the conductance behavior of ions in solution, especially when direct measurement is not feasible due to incomplete ionization.