Question

Which one of the following pairs will show positive deviation from Raoult's Law?

• A. Water - HCl
• B. Benzene - Methanol
• C. Water - $HNO_3$
• D. Acetone - Chloroform

Hint:

Positive deviation occurs when solute-solvent interactions are weaker than self-interactions, increasing vapor pressure.

Answer 1

The Correct Option is B

To determine which pair will show positive deviation from Raoult's Law, we need to understand what Raoult's Law is and what positive deviation implies.

Raoult's Law states that the vapor pressure of an ideal solution is directly proportional to the mole fraction of the solvent present in the solution. In a solution showing positive deviation from Raoult's Law, the observed vapor pressure is higher than predicted by Raoult's Law. This occurs when the interactions between unlike molecules are weaker than those among like molecules, leading to greater tendency for evaporation.

Let's analyze each pair: 

1. Water - HCl: This is a strong acid solution where hydrogen bonding occurs significantly between the components. Typically, this pair would show negative deviation due to strong intermolecular attractions.
2. Benzene - Methanol: Methanol can hydrogen bond, while benzene is non-polar and cannot hydrogen bond. The interaction between methanol and benzene is weaker than the hydrogen bonding between methanol molecules, leading to positive deviation.
3. Water - \(HNO_3\): Like water and HCl, water and nitric acid form a strong acid solution with significant hydrogen bonding, generally resulting in negative deviation.
4. Acetone - Chloroform: This pair commonly shows negative deviation from Raoult’s Law due to the presence of hydrogen bonding between chloroform and acetone, which is stronger than within the pure components.

Therefore, the correct answer is Benzene - Methanol, as the intermolecular forces between methanol and benzene are weaker than the forces within pure methanol, leading to positive deviation from Raoult's Law.