Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound \(\mathrm{A}\) of \(\mathrm{S}, \mathrm{Cl}\) and \(\mathrm{O}\) has a vapour density of \(67.5 .\) It reacts with water to form two acids and reacts with \(\mathrm{KOH}\) to form two salts \(\mathrm{B}\) and \(\mathrm{C}\). while \(\mathrm{B}\) gives white precipitate with \(\mathrm{AgNO}_{3}\) solution, \(\mathrm{C}\) gives white precipitate with \(\mathrm{BaCl}_{2}\) solution. Identify \(\mathrm{A}\) (a) \(\mathrm{SOCl}\) (b) \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{SO}_{3} \mathrm{Cl}\) (d) none of these

Step-by-Step Solution

Answer

Identify \( A \) as \( \text{SO}_2\text{Cl}_2 \).

Step 1: Calculate Molar Mass

The vapour density of compound \( A \) is given as 67.5. Vapour density is half of the molar mass. Therefore, the molar mass \( M \) can be calculated as:\[ M = 2 \times \text{vapour density} = 2 \times 67.5 = 135 \text{ g/mol} \]

Step 2: Analyze Reactions with Water and KOH

Compound \( A \) reacts with water to form two acids and with \( \text{KOH} \) to form two salts \( B \) and \( C \). This implies that \( A \) might be contributing two different acidic anions. The possible anions could be \( \text{Cl}^- \) and \( \text{SO}_4^{2-} \).

Step 3: Determine Reaction Behavior of Salts B and C

Salt \( B \) gives a white precipitate with \( \text{AgNO}_3 \), indicating the presence of \( \text{Cl}^- \) ion. Salt \( C \) gives a white precipitate with \( \text{BaCl}_2 \), indicating the presence of \( \text{SO}_4^{2-} \) ion.

Step 4: Identify Compound A from Given Options

Given the reactions and the molar mass, consider the options:- \( \text{SOCl}_2 \): Molar mass is 118 g/mol - Not a fit- \( \text{SO}_2\text{Cl}_2 \): Molar mass is 135 g/mol - A potential fit- \( \text{SO}_3\text{Cl} \): Molar mass does not matchCompound \( \text{SO}_2\text{Cl}_2 \) can form \( \text{SO}_3 \) and \( \text{HCl} \) when reacted with water, matching the behavior described for \( A \).

Key Concepts

Vapour Density

Vapour density is an essential concept in chemistry that helps identify compounds based on their molar mass. In simple terms, the vapour density of a substance is half of its molar mass. Knowing this relationship allows chemists to deduce the molar mass if they have the vapour density.
For compound A, given a vapour density of 67.5, we compute the molar mass as:

• The formula used is \( M = 2 \times \text{Vapour Density} \).
• In this case, \( M = 2 \times 67.5 = 135 \text{ g/mol} \).

Understanding this concept aids in narrowing down possible identities of unknown chemical compounds by ruling out those with incorrect molar masses.

Reaction with Water

When compound A reacts with water, it forms two different acids. This behavior suggests that compound A contains more than one type of acidic component. The reaction of a chemical compound with water often produces acids if the compound is a nonmetallic oxide or contains certain other acidic components.
In the case of our compound, the reaction with water results in the formation of distinct acids such as sulfuric acid (from sulfur-based anions) and hydrochloric acid (from chloride anions). This knowledge helps us understand how certain compounds interact with water, often leading to the production of specific recognizable products.

Salt Formation

Salt formation is a result of reactions between acids and bases. In the case of compound A, it reacts with potassium hydroxide (KOH), a strong base, to yield two salts, B and C. This occurs because the acids formed when A reacts with water can further react with bases like KOH to neutralize and form salts.

• Salt B precipitates as a white solid when treated with \( \text{AgNO}_3 \), indicating the presence of chloride ions because silver chloride (AgCl) forms.
• Salt C precipitates as a white solid in the presence of \( \text{BaCl}_2 \). This signifies the presence of sulfate ions, resulting in the formation of barium sulfate (BaSO4).

Through these reactions, we confirm the identity of the anions present, leading to an understanding of compound A's composition.

Precipitation Reactions

Precipitation reactions are employed to confirm the presence of specific ions within a solution. They involve the transformation of soluble ions into an insoluble solid, known as a precipitate, when mixed with another reagent.
In this exercise:

• When salt B forms a white precipitate with \( \text{AgNO}_3 \), we deduce that chloride ions (\( \text{Cl}^- \)) are present. Silver chloride (\( \text{AgCl} \)) is a classic precipitate in this scenario.
• Similarly, when salt C reacts with \( \text{BaCl}_2 \) to form a white precipitate, we identify this as a barium sulfate (\( \text{BaSO}_4 \)) formation, indicative of sulfate ions (\( \text{SO}_4^{2-} \)).

Such reactions are crucial analytical tools in the laboratory, allowing chemists to confirm the specific ions present in a sample.