Question

Identify the correct order of solubility of ${\text{N}}{{\text{a}}_2}{\text{S}},{\text{CuS and ZnS}}$ in aqueous medium.
A.${\text{CuS > ZnS > N}}{{\text{a}}_2}{\text{S}}$
B.${\text{ZnS > N}}{{\text{a}}_2}{\text{S > CuS }}$
C.${\text{N}}{{\text{a}}_2}{\text{S > CuS > ZnS }}$
D.${\text{N}}{{\text{a}}_2}{\text{S > ZnS > CuS }}$

Answer 1

For this question we must have the knowledge of the concept of solubility of ion in water. Solubility of ions depends on the hydration energy released when a substance is dissolved in water depending on their interaction and lattice energy. Higher the hydration energy released, higher will be solubility. Lower the lattice energy more is the solubility.

Complete step by step answer:
The maximum amount of solute that is dissolved in a given amount of solvent is termed as solubility. As we know water is a polar molecule and it has a dipole. Because of more electronegativity of oxygen it has slight negative charge and hydrogen has slight positive charge. Whenever we put any solvent in water it dissociates into its respective cation and anion. A cation is positively charged and oxygen in water has negative charge, so cation will interact with oxygen because opposite charges interact with each other. Similarly, the anion will interact with hydrogen or water. Due to this interaction with water energy is released called hydration energy. The more is hydration energy, the more is interaction of species with hydrogen or say more is solubility.
As anion is same in both the cases that is sulfide ion, so things will only depend on cation. Sodium, copper and zinc are from the third period and the size of the elements decreases along the period. According to Fajan’s rule, bigger the cation, more will be the ionic character and ionic salt is more soluble in water.
Hence the order of size of cation is ${\text{N}}{{\text{a}}^ + }{\text{ > C}}{{\text{u}}^ + }{\text{ > Z}}{{\text{n}}^{ + 2}}$
Same will be the order of solubility.

So option C is correct.

Note:
The bigger the cation is, more will be its volume and lesser will be its charge density, because volume is inversely proportional to the density. Lesser charge density means it will have low polarizing power and hence will have less covalent character or more ionic character.