Question

Which is least soluble?
A. $N{a}_{2}S$
B. $MgS$
C. $MgC{l}_2$
D. $NaCl$

Answer 1

Hint: In order to check for the solubility of compounds, their covalent character needs to be checked. The compound which has the maximum covalent character will be classified as the least soluble, and vice versa.

Complete step by step solution:
To answer the question, we must have a clear idea about Fajan’s rule. Fajan’s rule gives us the conditions for which covalent character can rise. According to it, covalent character is achieved when the size of the cation is small and the size of the anion is large. Now, let us check the covalent character in all the four given options.
Now, we see that the ions, which are generated from all the four options are $N{a}^{+},S^{2-},M{g}^{2+}$ and $C{l}^{-}$. We know that sodium is an alkali metal and magnesium is an alkaline earth metal. As atomic size decreases along a period , so we can say that $M{g}^{2+}$ has a smaller size than $N{a}^{+}$. So, as cation needs to be small, we cannot have sodium in the cation. Accordingly, options A and D are eliminated. Now, let us talk about the anion:
In options B and D, the cation is the same i.e magnesium. There is a fact that higher the charge, more is the covalent character. So, in our case, $S^{2-}$ has a more charge than $C{l}^{-}$. So option C is also eliminated and we get B as our final answer, based on the conditions of Fajan’s rule. So, out of the four given compounds ,we can predict that $MgS$ has the maximum amount of covalent character, which makes it the least soluble compound.

Hence, the answer is option (B).

Note: Compounds which are ionic, or have polarity are soluble in water. However, there are some compounds which have differing polarity, which gets cancelled out and hence they are not soluble. Even though non-polar compounds are not soluble in water, they are soluble in substances like fats and oils.