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 of the following pairs is an example of a positive deviation from Raoult's law, we need to understand the concepts of ideal solutions, positive deviations, and negative deviations.

### Step-by-Step Solution:

1. **Understanding Raoult's Law**:
Raoult's law states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction in the solution. For a solution of two volatile components A and B, the total vapor pressure (P_total) can be expressed as:
\[ P_{total} = P_A + P_B = P^0_A \cdot X_A + P^0_B \cdot X_B \]
where \(P^0_A\) and \(P^0_B\) are the vapor pressures of pure components A and B, and \(X_A\) and \(X_B\) are their mole fractions.

2. **Identifying Ideal Solutions**:
An ideal solution is one where the interactions between different molecules (A-B) are similar to those between like molecules (A-A and B-B). In such cases, the solution behaves according to Raoult's law.

3. **Positive Deviation**:
A positive deviation from Raoult's law occurs when the vapor pressure of the solution is higher than predicted by Raoult's law. This happens when the interactions between the different molecules (A-B) are weaker than the interactions between like molecules (A-A and B-B).

4. **Negative Deviation**:
Conversely, a negative deviation occurs when the vapor pressure is lower than predicted, which happens when the A-B interactions are stronger than A-A and B-B interactions.

5. **Analyzing the Given Options**:
- **Water + Hydrochloric Acid**: This shows negative deviation due to strong interactions.
- **Water + Nitric Acid**: This also shows negative deviation due to strong interactions.
- **Benzene + Methanol**: Benzene is nonpolar and methanol is polar. The weak interactions between these two lead to a positive deviation from Raoult's law.
- **Acetone + Chloroform**: This shows negative deviation due to strong hydrogen bonding interactions.

6. **Conclusion**:
From the analysis, the pair that exhibits a positive deviation from Raoult's law is **Benzene + Methanol**.

### Final Answer:
**Benzene + Methanol** is an example of a positive deviation from Raoult's law.