Question

Identify the correct order of solubility of $N{a_2}S$, $CuS$ and $ZnS$ in aqueous medium.
A: $CuS > ZnS > N{a_2}S$
B: $ZnS > N{a_2}S > CuS$
C: $N{a_2}S > CuS > ZnS$
D: $N{a_2}S > ZnS > CuS$

Answer 1

Solubility is a property of a substance. It is the ability of a solute to dissolve in a solvent. It is defined as the maximum amount of solute can be dissolved in a solvent at equilibrium. The solution, in which no more solute can be mixed at a particular temperature, is called saturated solution.

Complete step by step answer:
In this question, we have been given three compounds and we have to guess the correct order of solubility of these compounds. These compounds are, $N{a_2}S$, $CuS$ and $ZnS$. In these given compounds, anion is the same and cation is different. Also if we are comparing three compounds temperature, pressure and other factors will be common for all. Solubility of a substance depends on many factors. One of these factors is the size of the molecule. Molecules whose size is smaller are generally more soluble in a solvent because for small molecules, it is easy for solvent molecules to surround solute. If the molecule will be large, it will be difficult for solvent molecules to surround that molecule. Therefore, smaller will be the molecule, more will be the solubility. Among the given compounds, we have to find the cation (as anion is same in all the molecular size will depend on size of cation) which is smallest in size. Sodium lies above copper and zinc in periodic table and we know that moving down the group atomic size increases because the number of shells increases down the group. This means the size of sodium is the smallest (as zinc and copper are below sodium in periodic table). Along the period, size decreases because effective nuclear charge increases along the period (electrons go in the same shell along the period due to which shielding remains constant and attraction between electrons increases). As the effective nuclear charge increases, the hold of the nucleus on electrons increases and electrons go closer to the nucleus. As a result the size of the atom decreases. Atomic number of zinc is $30$ and of copper is $29$. In periodic table elements are arranged according to increasing atomic number. Therefore, zinc comes after copper as atomic number of zinc is more than that of copper and the size of zinc will be smaller than that of copper (along the period size decreases as explained above). So, correct order of size of cations will be,
$N{a^ + } < Z{n^{2 + }} < C{u^{2 + }}$
We know more the size of molecule less is the solubility and less is the size of molecule, more is the solubility. So, correct order of solubility will be,
$N{a_2}S > ZnS > CuS$
Therefore the correct answer is option D.

Note:
Saturated solution is that solution in which no more amount of solute can be added at a particular temperature but if we increase the temperature, more amount of solute can be added. Increase in temperature increases the solubility of solids in liquids.