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

Five ions are formed.

Step 1: Identify components in the compound

The compound in question is \(\mathrm{FeSO}_{4}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6\mathrm{H}_{2} \mathrm{O}\). It consists of \(\mathrm{FeSO}_{4}\), \((\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\), and \(6\mathrm{H}_{2}\mathrm{O}\). Recognize that \(\mathrm{H}_{2}\mathrm{O}\) represents water of crystallization and doesn't dissociate into ions upon dissolving.

Step 2: Dissociate \(\mathrm{FeSO}_{4}\) into ions

When \(\mathrm{FeSO}_{4}\) dissolves in water, it dissociates into two ions: \(\mathrm{Fe}^{2+}\) and \(\mathrm{SO}_{4}^{2-}\).

Step 3: Dissociate \((\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\) into ions

The compound \((\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\) dissociates into ions as follows: two \(\mathrm{NH}_{4}^{+}\) ions and one \(\mathrm{SO}_{4}^{2-}\) ion. These are the ions produced from this part of the compound.

Step 4: Count all ions formed

Counting the ions from Steps 2 and 3, we have: one \(\mathrm{Fe}^{2+}\), one \(\mathrm{SO}_{4}^{2-}\) from \(\mathrm{FeSO}_{4}\), two \(\mathrm{NH}_{4}^{+}\) ions, and one \(\mathrm{SO}_{4}^{2-}\) from \((\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\). In total, this sums to 1 \(\mathrm{Fe}^{2+} + 2 \mathrm{NH}_{4}^{+} + 2 \mathrm{SO}_{4}^{2-}\) ion types, which accounts for a total of 5 ions.

Key Concepts

Ion Dissociation

When certain compounds dissolve in water, they break apart into their respective ions in a process called ion dissociation. This happens because the water molecules are able to separate the ions from each other, allowing them to move freely in solution.
This is what occurs with the compound \( \mathrm{FeSO}_{4}\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\). When dissolved, its ionic components are released.

• \( \mathrm{FeSO}_{4} \) dissociates into \( \mathrm{Fe}^{2+} \) and \( \mathrm{SO}_{4}^{2-} \).
• \( (\mathrm{NH}_{4})_{2} \mathrm{SO}_{4} \) dissociates into two \( \mathrm{NH}_{4}^{+} \) ions and one \( \mathrm{SO}_{4}^{2-} \) ion.

To determine the total number of ions, we count each ion formed during the dissociation process. In this case, one \( \mathrm{Fe}^{2+} \), two \( \mathrm{NH}_{4}^{+} \), and two \( \mathrm{SO}_{4}^{2-} \) ions lead to the formation of five ions in total.

Water of Crystallization

Water of crystallization refers to the water molecules that are incorporated into the crystalline structure of a compound. They don't participate in ion dissociation. Instead, they are more like water molecules trapped inside the crystal.
In the compound \( \mathrm{FeSO}_{4}(\mathrm{NH}_{4})_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} \), the term \( 6 \mathrm{H}_{2} \mathrm{O} \) indicates the presence of six water molecules of crystallization. These molecules sit in the lattice and contribute to the stability and form of the compound. However, they don't dissociate into hydrogen and oxygen ions when the compound dissolves in water.
Therefore, while water of crystallization is essential to the compound's structure, it doesn't affect the number of ions formed during dissociation.

Ammonium Sulfate

Ammonium sulfate, represented as \( (\mathrm{NH}_{4})_{2} \mathrm{SO}_{4} \), is a vital component of the compound in question. It consists of ammonium ions \( \mathrm{NH}_{4}^{+} \) and sulfate ions \( \mathrm{SO}_{4}^{2-} \).
When ammonium sulfate dissolves in water, the compound separates into:

• Two \( \mathrm{NH}_{4}^{+} \) ions, carrying a positive charge.
• One \( \mathrm{SO}_{4}^{2-} \) ion, carrying a negative charge.

The positive and negative charges balance each other, maintaining an overall neutral solution. Ammonium sulfate is a classic example of an ionic compound, with its ions formed from a simple ammonium salt.

Ferrous Sulfate

Ferrous sulfate, written as \( \mathrm{FeSO}_{4} \), breaks down into iron (II) ions \( \mathrm{Fe}^{2+} \) and sulfate ions \( \mathrm{SO}_{4}^{2-} \) when it dissolves in water.
In the procedure of dissolving, each molecule of ferrous sulfate is separated by water molecules into:

• One \( \mathrm{Fe}^{2+} \) ion, which is a common type of iron ion.
• One \( \mathrm{SO}_{4}^{2-} \) ion, characterized by a two negative charge.

These dissociated ions allow ferrous sulfate to engage in chemical reactions, playing a significant role in various biological and chemical processes. Its ability to dissociate is what makes it useful in applications like water treatment and as a supplement in iron deficiency anemia.