Why is the molecular mass determined by measuring colligative property in case of some solutes is abnormal ?

Options

A. Due to association or dissociation of solute molecules
B. Due to insolubility of solute molecules
C. Due to decomposition of solute molecules
D. Due to large size of solute molecules.

Explanation

To understand why the molecular mass determined by measuring colligative properties can be abnormal for some solutes, we need to delve into the concept of colligative properties and how they relate to the behavior of solute molecules.

### Step-by-Step Solution:

1. **Understanding Colligative Properties**:
- Colligative properties are properties of solutions that depend on the number of solute particles in a given amount of solvent, rather than the identity of the solute. Common colligative properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

2. **Molecular Mass Determination**:
- The molecular mass of a solute can be determined using colligative properties by measuring how these properties change when a solute is added to a solvent. The expected molecular mass is calculated based on the number of particles that the solute contributes to the solution.

3. **Abnormal Molecular Mass**:
- In some cases, the measured molecular mass can be significantly different from the expected value. This discrepancy often arises due to the behavior of solute molecules in solution.

4. **Association and Dissociation**:
- Some solutes may associate (form larger aggregates) or dissociate (break into smaller particles) when dissolved in a solvent. For example:
- **Association**: Molecules of a solute can form dimers or larger aggregates, reducing the number of effective particles in solution.
- **Dissociation**: Ionic compounds can dissociate into their constituent ions, increasing the number of effective particles in solution.

5. **Effect on Molecular Mass Calculation**:
- If a solute associates, the number of particles in solution is less than expected, leading to an overestimation of the molecular mass. Conversely, if a solute dissociates, the number of particles is greater than expected, leading to an underestimation of the molecular mass.

6. **Conclusion**:
- Therefore, the abnormal molecular mass determined from colligative properties is primarily due to the association and dissociation of solute molecules. This alters the number of particles in solution, affecting the calculations based on colligative properties.

### Final Answer:
The molecular mass determined by measuring colligative properties can be abnormal for some solutes due to the association and dissociation of solute molecules.