Question

$HCl$ was passed through a solution of $CaC{l}_2,MgC{l}_2$ and $NaCl$. Which of the following compound(s) crystallise(s)?

• A. $NaCl,MgC{l}_2\text{and}CaC{l}_2$
• B. Only $NaCl$
• C. Only $MgC{l}_2$
• D. Both $MgC{l}_2\text{and}CaCl2$

Answer 1

The correct option is B Only $NaCl$

When $HCl$ is passed through the mixture $C{l}^{-}$ ion concentration increases.
Thus, ionic product $\left[N{a}^{+}\right]\left[C{l}^{-}\right]$ becomes higher than solubility product.
So, $NaCl$ will precipitate out.

$\text{Filtrate}\stackrel{HClgaspassed}{\to }\text{Pure NaCl}\downarrow \text{(common ion effect)}$

The hydration energies of group 2 ions are four or five times greater than group 1 ions.
Thus, calcium and magnesium chloride being more soluble than $NaCl$ will remain in the solution.

Hence, option (a) is correct.