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Chemistry Question 61 – NEET-UG 2025

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If the molar conductivity (Λm) of a 0.050 mol L−1 solution of a monobasic weak acid is 90 S cm2 mol−1, its extent (degree) of dissociation will be [Assume Λ+∘ = 349.6 S cm2 mol−1 and Λ−∘ = 50.4 S cm2 mol−1.]

The degree of dissociation of a weak electrolyte is the ratio of its molar conductivity at a given concentration to its limiting molar conductivity.

Step 1: Calculate Limiting Molar Conductivity (Λm∘)✦ Active

For a weak electrolyte, the limiting molar conductivity is the sum of the limiting molar conductivities of its cation and anion, according to Kohlrausch's Law.

Λm∘=Λ+∘+Λ−∘ Λm∘=349.6 S cm2 mol−1+50.4 S cm2 mol−1 Λm∘=400.0 S cm2 mol−1
💡 Teacher's Secret Hint

Kohlrausch's Law is essential for weak electrolytes, as their limiting molar conductivity cannot be determined by simple extrapolation from concentration-conductivity plots.

Step 2: Calculate Degree of Dissociation (α)○ Expand

The degree of dissociation (α) of a weak electrolyte is the ratio of its molar conductivity at a given concentration (Λm) to its limiting molar conductivity (Λm∘). The given concentration (0.050 mol L−1) is not directly used in this calculation for α but is the condition under which Λm is measured.

α=ΛmΛm∘ α=90 S cm2 mol−1400.0 S cm2 mol−1 α=0.225
💡 Teacher's Secret Hint

The degree of dissociation is a dimensionless quantity, representing the fraction of electrolyte molecules that have dissociated into ions at a specific concentration.

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