Electrochemistry (Class 12): Kohlrausch’s Law & Conductance, Made Simple
Understand molar conductivity, why it changes with dilution, and how Kohlrausch’s law helps you find values you can’t measure directly.
Key definitions
As concentration decreases, ions move more freely, so Λm increases and approaches a limiting value Λ°m at infinite dilution.
Kohlrausch’s law of independent migration
At infinite dilution, each ion contributes independently to conductivity:
This is why we can obtain Λ°m for a weak electrolyte like acetic acid by combining values from strong electrolytes.
Worked example
Q. Given Λ°m(HCl) = 426, Λ°m(CH₃COONa) = 91 and Λ°m(NaCl) = 126 (S cm² mol⁻¹), find Λ°m of acetic acid.
Common mistakes
- Mixing up conductivity (κ) and molar conductivity (Λm).
- Forgetting the 1000 factor in the Λm formula.
- Adding the wrong strong-electrolyte combination in Kohlrausch problems.
FAQs
Why does molar conductivity increase on dilution?
More water lets ions move independently, increasing conductivity per mole.
Why use Kohlrausch’s law for weak electrolytes?
Weak electrolytes never fully ionise, so Λ°m can’t be found by extrapolation — it is calculated from ions instead.
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