JEE Main 2020 · 6 September · morning · Single correct · +4 / −1
Among the sulphates of alkaline earth metals, the solubilities of BeSO₄ and MgSO₄ in water, respectively, are :
- (a) poor and poor
- (b) poor and high
- (c) high and poor
- (d) high and high
Answer and solution
Answer (d) high and high
Solution
Chemistry ma’am · 4 steps
The big sulphate ion keeps lattice energy steady, so tiny Be and Mg ions win on hydration energy.
- Two competing energies. A salt dissolves well when the energy released by hydrating its ions is more than its lattice enthalpy. So compare these two for each sulphate.
- Lattice enthalpy. The SO₄²⁻ ion is much larger than any of these cations. Its size dominates the ion spacing, so the lattice enthalpy stays nearly the same from BeSO₄ to BaSO₄.
- Hydration enthalpy. Hydration enthalpy depends on cation size. Small Be²⁺ and Mg²⁺ are hydrated very strongly, and this energy falls sharply for the larger Ca²⁺, Sr²⁺ and Ba²⁺.
Sulphate Hydration vs lattice Solubility in water BeSO₄ hydration far exceeds lattice high MgSO₄ hydration exceeds lattice high (Epsom salt) CaSO₄ nearly balanced sparingly soluble SrSO₄, BaSO₄ lattice wins practically insoluble - So. Both BeSO₄ and MgSO₄ are highly soluble in water, which is option (d).
The idea to keep. Group 2 sulphates get less soluble down the group: Be and Mg sulphates dissolve readily, while Ba sulphate is insoluble. Lattice enthalpy stays nearly constant, so hydration enthalpy decides.
Answer from the NTA final answer key. Solution by PrepWiser.