AIIMS 2005 Chemistry Colligative Properties Assertion Reason Question

To approach this question, we need to understand the concepts of colligative properties and how they relate to the molecular weight determination of solutes like acetic acid in different solvents.

1. Explanation of the Correct Answer

Assertion: The molecular weight of acetic acid determined by depression in freezing point method in benzene and water was found to be different.

This assertion is true because the freezing point depression (ΔTf) is a colligative property that depends on the number of solute particles in a solvent. The equation for freezing point depression is given by:

$$\Delta T_f = K_f \cdot m$$

where:

• $\Delta T_f$ is the depression in freezing point,
• $K_f$ is the freezing point depression constant of the solvent,
• $m$ is the molality of the solution.

When acetic acid is dissolved in water and benzene, the degree of dissociation and interaction with the solvent can differ significantly due to the polarity of the solvents, leading to different calculated molalities and thus different apparent molecular weights.

Reason: Water is polar and benzene is non-polar.

This reasoning is also true. The polarity of the solvent affects the solubility and the interactions between the solute (acetic acid) and the solvent. Acetic acid, a polar molecule, will dissociate and interact differently in polar solvents like water compared to non-polar solvents like benzene. This difference in interaction influences the observed freezing point depression values and subsequently leads to different determinations of molecular weight.

Since both the assertion and the reason are true, and the reason correctly explains why the molecular weights differ due to the nature of the solvents, the correct option is:

A) If both assertion and reason are true and reason is the correct explanation of assertion.

2. Clarification of Incorrect Options

• B) If both assertion and reason are true but reason is not the correct explanation of assertion: This option is incorrect because the reason directly explains why the molecular weights of acetic acid differ in the two solvents due to their differing polarities.

• C) If assertion is true but reason is false: This option is false as both parts are true.

• D) If both assertion and reason are false: This option is also incorrect as both the assertion and the reason are indeed true.

3. Relevant Concepts

Colligative Properties: Colligative properties depend on the number of solute particles rather than their identity. The key colligative properties include:

• Freezing point depression
• Boiling point elevation
• Vapor pressure lowering
• Osmotic pressure

4. Summary

In summary, the molecular weight of acetic acid differs when determined in benzene and water due to the polar nature of water versus the non-polar nature of benzene. This difference in polarity leads to varying interactions and solubilities of acetic acid, which is accurately captured by the freezing point depression method. The correct answer is A, as both the assertion and the reason are true and the reason accurately explains the assertion.