Addition of a non-volatile solute in a volatile ideal solvent

Options

A. Increases the vapoure pressure of the solvent Decreases the vapour pressure of the solvent
B. Decreases the vapoure pressure of the solvent
C. Decreases the boililng point of the solvent
D. Increases the freezing point of the solvent

Answer

### Step-by-Step Solution:

1. **Understanding the Components**:
- A non-volatile solute is a substance that does not evaporate easily and has a negligible vapor pressure.
- A volatile ideal solvent is a liquid that can evaporate easily and behaves ideally according to Raoult's Law.

2. **Raoult's Law**:
- Raoult's Law states that the vapor pressure of a solvent in a solution (P_solution) is directly proportional to the mole fraction of the solvent (X_solvent) in the solution.
- Mathematically, it can be expressed as:
\[ P_{\text{solution}} = X_{\text{solvent}} \cdot P^0_{\text{solvent}} \]
where \( P^0_{\text{solvent}} \) is the vapor pressure of the pure solvent.

3. **Effect of Adding Non-Volatile Solute**:
- When a non-volatile solute is added to the volatile solvent, the number of solvent molecules decreases in the solution because some of the volume is occupied by the solute.
- This results in a decrease in the mole fraction of the solvent (X_solvent).

4. **Calculating the New Vapor Pressure**:
- Since the mole fraction of the solvent decreases, according to Raoult's Law, the vapor pressure of the solution (P_solution) will also decrease.
- The new vapor pressure can be calculated using the modified mole fraction of the solvent after adding the solute.

5. **Conclusion**:
- The addition of a non-volatile solute to a volatile ideal solvent results in a decrease in the vapor pressure of the solvent. This is a colligative property, which depends on the number of solute particles in the solution rather than their identity.