Question

Which of the following mixtures show negative deviation from Raoult's law?

• A. II & III only
• B. III & IV only
• C. I & II only
• D. I & III only

Hint:

Mixtures with strong intermolecular forces (like H-bonding) between unlike molecules show negative deviation from Raoult's law.

Answer 1

The Correct Option is C

Step 1: Origin of negative deviation.
Negative deviation arises when A–B intermolecular attractions surpass A–A and B–B attractions.

Step 2: Examining the pairs.
- (I) CHCl₃ + (CH₃)₂CO: Strong hydrogen bonding produces negative deviation.
- (II) C₆H₅OH + C₆H₅NH₂: Hydrogen bonding exists, yielding negative deviation.
- (III) C₂H₅OH + (CH₃)₂CO: Some H-bonding but not definitively strong enough.
- (IV) C₆H₆ + C₆H₅CH₃: Dominated by weak van der Waals forces, essentially ideal.

Step 3: Selecting negative deviation cases.
Mixtures I and II clearly display negative deviation.

Step 4: Conclusion.
Therefore, the answer is I & II only.