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Correct order of limiting molar conductivity of the given ions at 298 K is

Conductance of Electrolytic Solutions
NEET
1

Br⁻ > Cl⁻ > OH⁻

2

OH⁻ > Cl⁻ > Br⁻

3

Cl⁻ > Br⁻ > OH⁻

4

OH⁻ > Br⁻ > Cl⁻

Solution:

Limiting molar conductivity (λ₀) of an ion depends on its mobility in the solution. The mobility is inversely proportional to the effective size of the hydrated ion.

  1. OH⁻ ion: The hydroxide ion (OH⁻) has exceptionally high limiting molar conductivity compared to other ions. This is due to the Grotthuss mechanism (or proton hopping mechanism) where the proton effectively 'jumps' from one water molecule to another, rather than the entire ion physically moving. This makes its effective mobility very high.

  2. Halide ions (Br⁻ and Cl⁻): For ions of similar charge, the smaller the bare ion, the greater its charge density. This leads to stronger hydration in aqueous solution, resulting in a larger hydrated radius. A larger hydrated radius means lower mobility and thus lower limiting molar conductivity.

    • Atomic size: Cl < Br

    • Hydrated size: Cl⁻ (hydrated) > Br⁻ (hydrated) (because Cl⁻ is smaller and gets more strongly hydrated)

    • Mobility: Br⁻ > Cl⁻

    • Limiting molar conductivity: λ₀(Br⁻) > λ₀(Cl⁻)

Combining these observations, the order of limiting molar conductivity is OH⁻ > Br⁻ > Cl⁻.

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