Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of ions formed on dissolving one molecule of \(\mathrm{FeSO}_{4}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) are (a) 3 (b) 4 (c) 5 (d) 6

Step-by-Step Solution

Answer

The number of ions formed is 4, so the correct choice is (b) 4.

Step 1: Understanding the Compound

The compound given is (FeSO₄)(NH₄)₂SO₄ · 6H₂O, known as ammonium iron(II) sulfate hexahydrate. It is a salt composed of iron(II) sulfate and ammonium sulfate with water molecules incorporated in its structure.

Step 2: Analyzing Dissociation

When the salt dissolves in water, it separates into its constituent ions. The dissolution can be represented as: FeSO₄(NH₄)₂SO₄ · 6H₂O → Fe²⁺ + SO₄²⁻ + 2NH₄⁺. Since water molecules do not dissociate into ions, they do not contribute to the total ion count.

Step 3: Counting the Ions

Upon dissociation, the compound breaks into one iron(II) ion (Fe²⁺), two ammonium ions (NH₄⁺), and two sulfate ions (SO₄²⁻). Therefore, the total number of ions produced is: 1 (Fe²⁺) + 2 (NH₄⁺) + 1 (SO₄²⁻) = 4 ions.

Key Concepts

Iron(II) Sulfate

Iron(II) sulfate is a chemical compound with the formula FeSO₄. When dissolved in water, it dissociates into iron (Fe²⁺) and sulfate (\(\text{SO}_4^{2-}\)) ions.

Understanding the dissociation process is key to grasping how this compound behaves in a solution.

• Iron Ions: The ferrous ion (\( ext{Fe}^{2+}\)) is produced, which is significant in various applications, such as water treatment and as a nutritional supplement.
• Sulfate Ions: The sulfate ion (SO₄²⁻) not only affects chemical reactions it participates in but also plays a role in environmental and industrial processes.

When iron(II) sulfate dissolves, it contributes one iron ion and one sulfate ion to the solution, making it essential to count these properly in exercises involving ionic compounds.

Ammonium Sulfate

Ammonium sulfate, with the formula \((\text{NH}_4)_2\text{SO}_4\), is another critical component of the compound in the exercise.

When ammonium sulfate dissolves in water, it dissociates into two ammonium ions (NH₄⁺) and one sulfate ion (SO₄²⁻).

• Ammonium Ions: Each ammonium molecule produces two NH₄⁺ ions. Ammonium is essential in agriculture as a source of nitrogen for plants.
• Sulfate Ions: It releases one ion (SO₄²⁻) per formula unit. The sulfate ions further emphasize the behavior of ionic compounds in solutions.

Combining ammonium sulfate's dissociation with that of iron(II) sulfate helps in understanding the complete ionization in certain chemical equations and exercises.

Hydrates

Hydrates are chemicals that contain water molecules bound within their crystal structure.

They are crucial for understanding how compounds like ammonium iron(II) sulfate hexahydrate behave.

• Water of Crystallization: These are water molecules that are integral in forming structure but do not affect ionic dissociation.
• Example: In ammonium iron(II) sulfate hexahydrate (\(\text{(FeSO}_4\text{)(NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}\)), the 6 water molecules contribute to the mass and the physical properties but do not dissociate into ions.

Hydrates are essential to consider in chemistry, particularly in determining the water content of compounds and understanding their structural properties. However, in exercises focusing on ion count, the hydrate part does not change the ionization process.