Question

Which of the following is a double salt:

• A. \(\mathrm{KCl} \cdot \mathrm{MgCl}_2 \cdot 6\mathrm{H}_2\mathrm{O}\)
• B. \(\mathrm{FeSO}_4 \cdot \left(\mathrm{NH}_4\right)_2\mathrm{SO}_4 \cdot 6\mathrm{H}_2\mathrm{O}\)
• C. \(\mathrm{KAl}\left(\mathrm{SO}_4\right)_2 \cdot 12\mathrm{H}_2\mathrm{O}\)
• D. All of above

Hint:

For double salts:

- Contain multiple cations/anions, ionize completely in solution.

- Examples include alums, Mohr’s salt, carnallite.

- Distinguish from complex salts with coordination ions.

Answer 1

The Correct Option is D

A double salt contains two or more different cations or anions in a definite stoichiometric ratio, forming a crystalline compound that dissociates into its constituent ions in solution. Unlike complex salts, double salts do not retain complex ions in solution.
Step 1: Define double salts
Double salts ionize completely in water into individual ions (e.g., \({K^+}, {Mg^{2+}}\)), unlike coordination compounds with complex ions (e.g., \({[Fe(CN)6]^{4-}}\)).
Step 2: Analyze options
- (A) \(\mathrm{KCl} \cdot \mathrm{MgCl}_2 \cdot 6\mathrm{H}_2\mathrm{O}\): Carnallite, a double salt containing \({K^+}\), \({Mg^{2+}}\), and \({Cl^-}\) ions, with water of crystallization. Ionizes as \({K^+}, {Mg^{2+}}, {Cl^-}\).
- (B) \(\mathrm{FeSO}_4 \cdot \left(\mathrm{NH}_4\right)_2\mathrm{SO_4 \cdot 6\mathrm{H}_2\mathrm{O}\)}: Mohr’s salt, a double salt with \({Fe^{2+}}\), \({NH4^+}\), and \({SO4^{2-}}\) ions. Ionizes completely.
- (C) \(\mathrm{KAl}\left(\mathrm{SO}_4\right)_2 \cdot 12\mathrm{H_2\mathrm{O}\)}: Potash alum, a double salt with \({K^+}\), \({Al^{3+}}\), and \({SO4^{2-}}\) ions. Ionizes as individual ions.
Step 3: Conclusion
All are double salts, matching option (D).