Question

Why is Sodium Sulphate soluble in water whereas Barium Sulphate is sparingly soluble in water? (Question may have multiple correct answer)
A) The Hydration Energy of $N{a_2}S{O_4}$ is more than its Lattice Energy.
B) The Lattice Energy of $BaS{O_4}$ is more than its Hydration Energy.
C) The Lattice Energy has no role to play in solubility.
D) The Lattice Energy of $N{a_2}S{O_4}$ is more than its Hydration Energy.

Answer 1

Here we recall the concept of Solubility of ionic salt in water.
Solubility depends on Lattice energy and Hydration energy.
Smaller the size of cation, Higher the Hydration energy for cation, and more is solubility.
Greater the size of cation, Higher the Lattice energy, and less is solubility.

Complete answer:
Hydration Energy: It is the amount of energy released when one mole of ions undergo hydration.
Lattice Energy: It is the energy released when ions are combined to make a compound.
The Solubility of ionic solids depends on Hydration energy and Lattice energy.
Due to smaller size of alkaline earth metals cations compared to corresponding alkali metal cations, alkaline earth metal cations are strongly attracted to anions, thus Lattice energy is high and Hydration energy is less.
Sodium is alkali metal, while Barium is alkaline earth metal, so Sodium Sulphate has higher hydration energy than Lattice energy. And Barium sulphate has higher Lattice energy than Hydration energy.
Thus, sodium sulphate is soluble in water while Barium sulphate is sparingly soluble in water.

So, Option (A) and (B) both are correct.

Additional Information: For alkali metal cation, size is greater than alkaline earth metal due to Effective nuclear charge. Effective nuclear charge is high as the number of protons are more in corresponding alkaline earth metals, which attracts the electrons more strongly and thus reduces the size of Alkaline earth metals cation.

Note: Comparison of size of different cations can be done when anions are the same. Size affects Hydration energy and Lattice energy and thus affects solubility of ionic solids.