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  4. If Na+ ion is larger than Mg2 + ion and S2 - is larger than Cl- ion, which of the following will be least soluble in water?

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Q. If $Na^{+}$ ion is larger than $Mg^{2 +}$ ion and $S^{2 -}$ is larger than $Cl^{-}$ ion, which of the following will be least soluble in water?

NTA AbhyasNTA Abhyas 2022

Solution:

Higher is the lattice energy lower is the solubility. Out of the four combinations possible, the lattice energy of $MgS$ (bi-bivalent ionic solid) is higher than those of $Na_{2}S, \, MgCl_{2}$ and $NaCl$ (univalent ionic solids) and hence, $MgS$ is the least soluble. Thus, due to higher charge/ion, the lattice formed between $Mg^{2 +}$ and $S^{2 -}$ is very strong.
As lattice energy overweighs the hydration energy here, the salt is least soluble.