What is the colour of Mohr's salt?

Green

The correct answer is option 3. Green.

Mohr's salt, which is also known as ferrous ammonium sulfate \(((NH_4)_2Fe(SO_4)_2·6H_2O)\), is a coordination compound containing iron. It is named after the German chemist Karl Friedrich Mohr.

The color of Mohr's salt is typically pale green. So, the correct option is 3. Green

Additional Information:

Mohr's salt, chemically known as ferrous ammonium sulfate hexahydrate \(((NH_4)_2Fe(SO_4)_2·6H_2O)\), is a double salt containing iron(II) ions \((Fe^{2+})\), ammonium ions \((NH_4^+)\), and sulfate ions \((SO_4^{2-})\). This compound is widely used in various applications, including analytical chemistry, photography, and as a reducing agent in chemical reactions.

Now, let's delve into the details of the color and properties of Mohr's salt:

1. Color:  Mohr's salt typically appears as pale green crystals or powder. The green color arises from the complexation of the iron(II) ions with sulfate ions and water molecules in the crystal lattice. The specific shade of green may vary depending on factors such as impurities and crystal structure.

2. Chemical Composition: Mohr's salt has the chemical formula \((NH_4)_2Fe(SO_4)_2·6H_2O\), indicating its composition of two ammonium ions, one iron(II) ion, two sulfate ions, and six water molecules per formula unit. The presence of water molecules in the crystal structure contributes to its hydrated form.

3. Crystal Structure:  Mohr's salt crystallizes in the monoclinic crystal system. Its crystal structure consists of layers of \(Fe(H_2O)_6^{2+}\) octahedra and \(SO_4^{2-}\) tetrahedra, with ammonium ions occupying the interlayer spaces. This arrangement contributes to the stability and solubility of the compound.

4. Applications:

Analytical Chemistry:  Mohr's salt is commonly used as a primary standard for volumetric analysis, particularly in titrations involving permanganate ions (MnO4^-) or dichromate ions \((Cr_2O_7^{2-})\). It serves as a source of standardized iron(II) ions for redox titrations.

Photography: Mohr's salt has historically been used in photographic developers due to its reducing properties. It can act as a mild reducing agent, facilitating the reduction of silver ions \((Ag^+)\) to metallic silver \((Ag)\) in photographic emulsions.

Chemical Synthesis: Mohr's salt finds applications as a reducing agent in various chemical reactions, including the production of other iron compounds and in organic synthesis.

5. Solubility and Stability:  Mohr's salt is readily soluble in water, forming a green-colored solution. However, it is unstable in air and can oxidize to form ferric iron \((Fe^{3+})\) compounds over time. Therefore, it should be stored in airtight containers to prevent deterioration.

In summary, Mohr's salt is a pale green-colored compound with diverse applications in analytical chemistry, photography, and chemical synthesis. Its color, composition, crystal structure, and properties make it a valuable reagent in various laboratory and industrial processes.