Mohr's salt is ______ in colour.

Green

The correct answer is option 3. Green.

Mohr's salt, scientifically known as ferrous ammonium sulphate, is a coordination compound with the chemical formula \([Fe(NH_4)_2(SO_4)_2 \cdot 6H_2O]\). It is a hydrated double salt containing iron, ammonium, and sulfate ions. The complex is commonly used in titration experiments, particularly for determining the concentration of permanganate ions (MnO₄⁻) in a solution.

Let us break down the composition and color of Mohr's salt:

1. Composition:

The chemical formula \([Fe(NH_4)_2(SO_4)_2 \cdot 6H_2O]\) indicates that the complex consists of iron (Fe), ammonium (\(NH_4^+\)), sulfate (\(SO_4^{2-}\)), and water (\(H_2O\)) molecules.

The ammonium ions and sulfate ions form coordination complexes with the iron ion in a specific arrangement.

2. Color:

Mohr's salt is typically pale green in color. The green color is associated with the specific arrangement of ligands (ammonium and sulphate ions) around the iron ion in the complex.

The absorption of certain wavelengths of light by the complex gives rise to its characteristic color.

3. Use in Titration:

Mohr's salt is commonly used as a primary standard in titration experiments, particularly for titrating solutions containing permanganate ions (\(MnO_4^-\)).

The reaction involves the oxidation of ferrous ions (\(Fe^{2+}\)) in Mohr's salt to ferric ions (\(Fe^{3+}\)) by permanganate ions (\(MnO_4^-\)).

The endpoint of the titration is determined by a color change from pale pink (due to the permanganate ion) to a faint straw color, indicating the complete reaction.

Overall, Mohr's salt is a versatile compound in analytical chemistry, known for its distinctive pale green color and its role as a reliable primary standard in titration procedures.