Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a complex 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) \(4 \mathrm{KCN} \cdot \mathrm{Fe}(\mathrm{CN})\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

The complex salt is (b) \( \mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} \).

Step 1: Understanding Complex Salts

A complex salt, also known as a coordination compound, contains a central metal atom or ion that is bonded to a group of molecules or ions, called ligands, through coordinate covalent bonds. These compounds have complex ions as part of their structure.

Step 2: Analyzing Option (a)

Option (a) is \( \mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This is a mixture of potassium chloride and magnesium chloride with water molecules. There are no complex ions involved, so it is not a complex salt.

Step 3: Analyzing Option (b)

Option (b) is \( \mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This is known as Mohr's salt, which contains \( \mathrm{Fe}^{2+} \) coordinated with \( \mathrm{SO}_4^{2-} \) and \( \mathrm{NH}_{4}^{+} \). This forms a complex salt because of the presence of complex ions.

Step 4: Analyzing Option (c)

Option (c) is \( 4 \mathrm{KCN} \cdot \mathrm{Fe}(\mathrm{CN}) \). This is an incomplete and incorrect representation of a compound. However, if this was intended to be \( \mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right] \), it forms a complex ion, known as potassium ferrocyanide, which is a complex salt.

Step 5: Analyzing Option (d)

Option (d) is \( \mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O} \). This is known as potash alum, a double salt, but not a complex salt as it does not have a central metal atom bonded to ligands.

Step 6: Choosing the Correct Option

Considering the definition of a complex salt and the analysis of each option, option (b), \( \mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} \), is a complex salt because it involves complex ion formation.

Key Concepts

Coordination Compounds

Coordination compounds, also known as complex salts, contain a central metal atom or ion surrounded by other molecules or ions, known as ligands. These ligands are attached to the central atom through coordinate covalent bonds. This unique bonding arrangement leads to the formation of complex ions, which are a key characteristic feature of coordination compounds.
This category of chemical compounds exhibits fascinating properties. Coordination compounds are often vibrant in color and have distinct magnetic and optical attributes. Their structures can also be geometric or resemble various polyhedra, depending on the number and arrangement of ligands.

• Central Metal Ion: The metal ion acts as the central point to which ligands are attached.
• Ligands: Molecules or ions that donate a pair of electrons to the metal to form a coordinate bond.
• Coordination Number: The total number of ligand attachments to the metal ion.

Coordination compounds are used in various applications, including the synthesis of materials and as catalysts in chemical reactions.

Complex Ions

Complex ions are essential components of coordination compounds. These ions consist of a central metal atom or ion surrounded by a set of ligands. A unique feature of complex ions is the presence of coordinate covalent bonds, where the ligands contribute a pair of electrons to form a bond with the metal.

For example, in the ion \([ ext{Fe(CN)}_6]^{4-}\), the iron (Fe) is the central metal, and each cyanide ion (CN) acts as a ligand. The cyanide ions donate electrons to form stable bonds with the iron. This forms a complex ion with distinct properties.

• Formation of complex ions often leads to stable and specific structures.
• The charge of a complex ion is determined by the sum of charges on the central metal and its attached ligands.

Complex ions play a significant role in various chemical characteristics and reactions, influencing the solubility and color of the compounds they are in.

Mohr's Salt

Mohr's salt is an example of a coordination compound known as a complex salt. It is chemically represented as \(\text{FeSO}_4 \cdot \left(\text{NH}_4\right)_2 \text{SO}_4 \cdot 6 \text{H}_2 \text{O}\). Mohr's salt features ferrous ions (Fe\(^{2+}\)) forming a complex with sulfate (\(\text{SO}_4^{2-}\)) and ammonium ions (\(\text{NH}_4^{+}\)).
This formulation showcases how Mohr's salt includes both coordination bonds and typical ionic bonds. It highlights the versatility of complex salt structures in forming stable interactions with multiple substances.

• Mohr's salt is commonly used in titration processes due to its stability and clarity.
• Its crystalline structure is long-lasting, given the presence of water of crystallization.

This stability makes Mohr's Salt an excellent choice for laboratory applications and experiments.

Double Salts

Double salts are mixtures of two different salts that crystallize together but dissociate back into their constituent ions in water. Unlike complex or coordination compounds, double salts do not maintain their identity in solution.
Potash alum, represented as \(\text{K}_2 \text{SO}_4 \cdot \text{Al}_2\left(\text{SO}_4\right)_3 \cdot 24 \text{H}_2 \text{O}\), is a classic example of a double salt. In water, it dissociates into potassium ions, aluminum ions, and sulfate ions.

• Double salts are not involved in the complex ion formation seen in coordination compounds.
• They play a crucial role in industries, particularly in water purification and in dyeing fabrics.

Understanding the difference between double salts and complex salts is crucial in grasping the fundamentals of inorganic chemistry.