JEE Main 2022 · 25 June · morning · Single correct · +4 / −1
Which one of the following alkaline earth metal ions has the highest ionic mobility in its aqueous solution?
- (a) Be²⁺
- (b) Mg²⁺
- (c) Ca²⁺
- (d) Sr²⁺
Answer and solution
Answer (d) Sr²⁺
Solution
Chemistry ma’am · 3 steps
In water, an ion does not move bare: it drags its shell of water molecules along with it.
- Ions travel hydrated. In aqueous solution each M²⁺ ion is surrounded by tightly held water molecules. Its mobility depends on the size of this hydrated ion, not on its bare radius.
- Smaller means more water. All four ions carry the same +2 charge, so the smallest ion has the highest charge density. It pulls in the most water, so it ends up with the largest hydrated radius and moves the slowest.
Ion Bare radius Hydration enthalpy Hydrated size Mobility Be²⁺ 31 pm -2494 kJ mol⁻¹ largest lowest Mg²⁺ 72 pm -1921 kJ mol⁻¹ large low Ca²⁺ 100 pm -1577 kJ mol⁻¹ smaller higher Sr²⁺ 118 pm -1443 kJ mol⁻¹ smallest highest - So. Sr²⁺ is the largest bare ion and the least hydrated, so its hydrated ion is the smallest and it has the highest ionic mobility. The answer is (d).
The idea to keep. Down a group, bare ions get bigger but hydrated ions get smaller. So in water, ionic mobility and conductance increase down the group.
Answer from the NTA final answer key. Solution by PrepWiser.