Question

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

• A. Water - HCl
• B. Benzene - Methanol
• C. Water - HNO₃
• D. Acetone - Chloroform

Answer 1

The Correct Option is B

• A positive deviation from Raoult's Law occurs when the vapor pressure of the solution is higher than expected. This happens when the intermolecular forces between the components of the solution are weaker than the forces between the molecules of the individual components in their pure states.
• Benzene and methanol form a mixture that exhibits positive deviation because the hydrogen bonding between methanol molecules is stronger than the interactions between methanol and benzene molecules. As a result, the overall intermolecular forces are weaker in the solution, causing an increase in vapor pressure.

For the other pairs:

• Water - HCl and Water - HNO₃: These form strong hydrogen bonds between components, leading to a negative deviation from Raoult's Law.
• Acetone - Chloroform: This pair also forms strong interactions between molecules, leading to a negative deviation.

Thus, the pair that shows positive deviation from Raoult's Law is (B) Benzene - Methanol.

Answer 2

Raoult’s Law states that the partial vapor pressure of each volatile component in a mixture is proportional to its mole fraction. Positive deviation occurs when the intermolecular forces between the components of the mixture are weaker than those between the molecules of the individual pure substances. Benzene and methanol show a positive deviation because of their differing intermolecular interactions.