JEE Main 2021 · 17 March · morning · Single correct · +4 / −1
The correct order of conductivity of ions in water is :
- (a) K⁺ > Na⁺ > Cs⁺ > Rb⁺
- (b) Cs⁺ > Rb⁺ > K⁺ > Na⁺
- (c) Na⁺ > K⁺ > Rb⁺ > Cs⁺
- (d) Rb⁺ > Na⁺ > K⁺ > Li⁺
Answer and solution
Answer (b) Cs⁺ > Rb⁺ > K⁺ > Na⁺
Solution
Chemistry ma’am · 3 steps
In water, an ion moves with its hydration shell, so the smallest bare ion ends up the slowest.
- Ions carry water. In water, every cation pulls a shell of water molecules around itself and drags it along as it moves. What decides its speed is the size of this hydrated ion, not the bare ion.
- Small means more water. A smaller bare ion has a higher charge density, so it holds more water molecules more tightly. Going down the group from Na⁺ to Cs⁺, the bare ion grows and hydration gets weaker.
Ion Bare ion size Extent of hydration Hydrated size Mobility and conductivity Na⁺ smallest most largest lowest K⁺ larger less smaller higher Rb⁺ larger still less still smaller still higher still Cs⁺ largest least smallest highest - So. A smaller hydrated ion moves faster and conducts better, so the order is Cs⁺ > Rb⁺ > K⁺ > Na⁺, which is option (b).
The idea to keep. For ionic conductivity in water, think of the hydrated ion. The smaller the bare ion, the bigger its water shell, and the slower it moves.
Answer from the NTA final answer key. Solution by PrepWiser.