Inorganic Preparations: Double Salts

Formula / Core Reaction:

$$\text{FeSO}_4 + (\text{NH}_4)_2\text{SO}_4 + 6\text{H}_2\text{O} \rightarrow \text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O} \quad \text{(Mohr's Salt)}$$

$$\text{K}_2\text{SO}_4 + \text{Al}_2(\text{SO}_4)_3 + 24\text{H}_2\text{O} \rightarrow \text{K}_2\text{SO}_4 \cdot \text{Al}_2(\text{SO}_4)_3 \cdot 24\text{H}_2\text{O} \quad \text{(Potash Alum)}$$

Concept: Equimolar aqueous solutions of constituent single salts are mixed and crystallized to yield double salts that completely dissociate into their constituent ions in aqueous solution.

Notation / Key Observation:

• Mohr's Salt: Ferrous ammonium sulfate hexahydrate (Light green crystals);
• Potash Alum: Potassium aluminum sulfate 24-hydrate (Colorless monoclinic/octahedral crystals).