State Kohlrausch's law of independent migration of ions. How is it used to find (i) Λ°m of a weak electrolyte and (ii) its degree of dissociation? Show how Λm varies with √c for a strong and a weak electrolyte.

1. 1 · State the law. At infinite dilution, Λ°m of an electrolyte = sum of the limiting molar conductivities of its cation and anion: Λ°m = ν₊λ°₊ + ν₋λ°₋.
2. 2 · Draw the graph. Λm (y-axis) against √c (x-axis): a gently falling straight line for KCl, extended to meet the y-axis at Λ°m; a steep curve for CH₃COOH.
3. 3 · Application (i). Λ°m(CH₃COOH) = Λ°m(HCl) + Λ°m(CH₃COOK) − Λ°m(KCl) = 425.9 + 114.4 − 149.8 = 390.5 S cm² mol⁻¹.
4. 4 · Application (ii). α = Λm ÷ Λ°m, and then Ka = cα² ÷ (1 − α).
5. 5 · Explain the shapes. A strong electrolyte is fully ionised, so Λm rises slowly. A weak one ionises more on dilution, so Λm rises steeply.
6. 6 · Close in one line. So Kohlrausch's law gives Λ°m exactly where the graph cannot.

Scoring tip: Show the ion check, (H⁺ + Cl⁻) + (K⁺ + CH₃COO⁻) − (K⁺ + Cl⁻), in one line. It proves your combination is right.