Kohlrausch's Law of Independent Migration

CHEMISTRY SLET QUESTIONS

Kohlrausch's law of independent migration of ions is primarily used to determine the limiting molar conductivity (Λ_m^o) of:

• (A) Strong electrolytes only
• (B) Weak electrolytes
• (C) Non-electrolytes
• (D) Solutions at high concentration

Correct Answer: (B) Weak electrolytes

Explanation:

While Kohlrausch's Law applies to all electrolytes, its most significant practical application is calculating the Λ_m^o for weak electrolytes (like acetic acid).

The Problem: For weak electrolytes, the graph of Λ_m vs √c does not approach the y-axis linearly, making it impossible to determine the limiting molar conductivity through simple extrapolation.

The Solution: Kohlrausch's Law states that at infinite dilution, each ion makes a definite contribution toward molar conductivity. Therefore, we can calculate Λ_m^o for a weak electrolyte by summing the individual ionic conductivities of its constituent ions, which are often derived from strong electrolytes.

Example: Λ_m^o(CH₃COOH) = Λ^o(CH₃COO⁻) + Λ^o(H⁺)