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ChemistryMediumClass 12

Conductance — Λm, Kohlrausch's law

Electrochemistry

9

JEE Qs

8%

Hard

60

min

Master unit conversions and understand the qualitative differences in conductance behavior between strong and weak electrolytes with dilution.

🧮 Key Formulas

G = 1/R
κ = G * (l/A) = G * G_cell (where G_cell is cell constant)
Λm = (κ * 1000) / C (if κ in S cm^-1, C in mol L^-1, Λm in S cm^2 mol^-1)
Λm = κ / C (if κ in S m^-1, C in mol m^-3, Λm in S m^2 mol^-1)
Λm = Λm^0 - A * sqrt(C) (Debye-Hückel-Onsager equation for strong electrolytes)
Λm^0 = ν+ * λ+^0 + ν- * λ-^0 (Kohlrausch's Law of Independent Migration of Ions)
α = Λm / Λm^0 (Degree of dissociation for weak electrolytes)
K_a = (C * α^2) / (1 - α) (Ostwald's Dilution Law for weak acid)

✅ Key Points for JEE

  • 1Unit consistency is paramount in calculations; ensure κ (S cm⁻¹ or S m⁻¹), C (mol L⁻¹ or mol m⁻³), and Λm (S cm² mol⁻¹ or S m² mol⁻¹) are correctly interconverted.
  • 2Specific conductance (κ) generally decreases with dilution for both strong and weak electrolytes, while molar conductivity (Λm) increases with dilution for both.
  • 3Strong electrolytes show a linear decrease in Λm with sqrt(C) (Debye-Hückel-Onsager), whereas weak electrolytes show a very sharp increase in Λm at extreme dilution due to increased degree of dissociation.
  • 4Kohlrausch's Law allows calculation of Λm^0 (limiting molar conductivity) for weak electrolytes by algebraically combining Λm^0 values of suitable strong electrolytes.
  • 5The degree of dissociation (α) for a weak electrolyte can be determined from its molar conductivity at a given concentration and its limiting molar conductivity (α = Λm / Λm^0).

⚠️ Common Mistakes

  • ✕Errors in unit conversions (e.g., L to cm³, S cm⁻¹ to S m⁻¹, mol L⁻¹ to mol m⁻³), leading to incorrect magnitude of Λm.
  • ✕Confusing the trends of specific conductance (κ) and molar conductivity (Λm) with dilution for strong vs. weak electrolytes.
  • ✕Incorrectly applying Kohlrausch's law for calculating Λm at finite concentration instead of limiting molar conductivity (Λm^0).
  • ✕Misinterpreting the significance of the steep rise in Λm for weak electrolytes at high dilution compared to strong electrolytes.

📝 Practice Questions

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NCERT Chapters

  • Class 12 Chemistry Part 1 Ch 3: Electrochemistry