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For a strong electrolyte AB in water at 25 deg C, the molar conductivity Lambdaₘ varies with concentration as: at C = 0.25 mol/L, Lambdaₘ = 160 S cm²/mol; at C = 1 mol/L, Lambdaₘ = 150 S cm²/mol. Using the Debye-Huckel-Onsager (Kohlrausch) relation, find the limiting molar conductivity Lambda°m (in S cm² mol⁻¹).
- 160
- 150
- 170
- 180
Correct answer: 170
Solution
Kohlrausch's law for strong electrolytes: Lambdaₘ = Lambda°m - K*sqrt(C). At C = 0.25, sqrt(C) = 0.5: 160 = Lambda°m - 0.5K. At C = 1, sqrt(C) = 1: 150 = Lambda°m - K. Subtracting: 160 - 150 = (Lambda°m - 0.5K) - (Lambda°m - K) => 10 = 0.5K => K = 20. Then Lambda°m = 150 + K = 150 + 20 = 170 S cm² mol⁻¹.
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