Which of the following pairs is an example of a positive deviation from Raoult's law ?

Options

A. Water - hydrchloric acid
B. Water - nitric acid
C. Benzene - methanol
D. Acetone - chloroform

Answer

To determine which pair is an example of a positive deviation from Raoult's law, we need to understand what positive deviation means in the context of solutions.

### Step-by-Step Solution:

1. **Understanding Raoult's Law**:
- Raoult's law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent in the solution.
- For ideal solutions, the interactions between different molecules are similar to those between like molecules.

2. **Identifying Positive Deviation**:
- Positive deviation occurs when the vapor pressure of the solution is higher than what Raoult's law predicts. This typically happens when the interactions between different components in the solution are weaker than the interactions between like molecules.
- In such cases, the presence of one component reduces the overall intermolecular forces, leading to an increase in vapor pressure.

3. **Analyzing the Given Options**:
- We need to analyze the interactions of the components in each pair provided in the options (A, B, C, D).
- For example, if we consider benzene and methanol:
- Benzene has weak van der Waals forces, while methanol has strong hydrogen bonding.
- When mixed, benzene disrupts the hydrogen bonding among methanol molecules, leading to an increase in vapor pressure. This results in a positive deviation from Raoult's law.

4. **Conclusion**:
- Based on the analysis, the pair that shows a positive deviation from Raoult's law is **benzene and methanol**.
- Therefore, the correct option is **C**.

### Final Answer:
The correct answer is **C: Benzene and Methanol**.