Identify the mixture that shows positive deviations from Raoult's Law

Solution

Chemistry ma’am · 2 steps

If mixing makes A–B attractions weaker than A–A and B–B, vapour pressure rises: positive deviation.

1. What positive means. In a positive deviation the new A–B attractions are weaker than the A–A and B–B attractions in the pure liquids. Molecules escape more easily, so the vapour pressure is higher than Raoult's law predicts. If mixing creates stronger attractions, often a new hydrogen bond, the deviation is negative.

   Mixture	What happens on mixing	Deviation
   (CH₃)₂CO + CS₂	Polar acetone molecules lose their strong dipole–dipole attractions to non-polar CS₂. Only weak acetone–CS₂ attractions form.	positive
   (CH₃)₂CO + C₆H₅NH₂	The N–H of aniline forms a hydrogen bond with the C=O of acetone.	negative
   CHCl₃ + C₆H₆	The C–H of chloroform is attracted to the π electron cloud of benzene.	negative
   CHCl₃ + (CH₃)₂CO	The C–H of chloroform forms a hydrogen bond with the C=O of acetone.	negative

2. So. Only acetone with carbon disulphide weakens the attractions between molecules on mixing. The answer is (a).

The idea to keep. Ask one thing: does mixing make a new attraction, like a hydrogen bond, or break old ones? A new attraction gives negative deviation. Broken attractions give positive deviation.

Answer from the NTA final answer key. Solution by PrepWiser.