JEE Main 2025 · 3 April · morning · Single correct · +4 / −1
Correct order of limiting molar conductivity for cations in water at 298 K is :
- (a) H⁺>Na⁺>K⁺>Ca²⁺>Mg²⁺
- (b) H⁺>Ca²⁺>Mg²⁺>K⁺>Na⁺
- (c) Mg²⁺>H⁺>Ca²⁺>K⁺>Na⁺
- (d) H⁺>Na⁺>Ca²⁺>Mg²⁺>K⁺
Answer and solution
Answer (b) H⁺>Ca²⁺>Mg²⁺>K⁺>Na⁺
Solution
Chemistry ma’am · 4 steps
Think about the hydrated ion, not the bare ion: a small ion holds more water and moves slower.
- H+ is special. H⁺ does not travel through water as one particle. It hops from one water molecule to the next by the Grotthuss mechanism. That makes it far faster than any other cation, so it comes first.
- Charge counts. Limiting molar conductivity is per mole of ions. A mole of Ca²⁺ or Mg²⁺ carries twice the charge of a mole of K⁺ or Na⁺. So the 2+ ions sit above the 1+ ions.
- Hydration decides. Within each charge group, the smaller bare ion attracts water more strongly. It drags a bigger hydration shell and moves more slowly. Mg²⁺ is smaller than Ca²⁺, and Na⁺ is smaller than K⁺, so both are the slower ones.
Cation Λ°ₘ at 298 K (S cm² mol⁻¹) Reason for its place H⁺ 349.6 proton hopping Ca²⁺ 119.0 2+ charge, less hydrated than Mg²⁺ Mg²⁺ 106.0 2+ charge, but heavily hydrated K⁺ 73.5 1+ charge, lightly hydrated Na⁺ 50.1 1+ charge, more hydrated - So. The order is H⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺, which is option (b).
The idea to keep. For ionic conductance in water, rank by charge first, then by hydrated size: the smaller bare ion is more hydrated and slower. H⁺ and OH⁻ always top their lists.
Answer from the NTA final answer key. Solution by PrepWiser.