Raoult’s Law - Practice Questions & MCQ

These solutions have vapour pressure less than that predicted by Raoult’s law for the entire range of composition.

This happens when the new solute-solvent interactions are stronger than the interactions in the pure components. Since the newly formed forces are stronger than the existing forces, heat is liberated. Hence, the enthalpy change in the mixing of these solutions is negative i.e

The change in the volume during the mixing process is positive i.e.. For example, if 1 litre solutions each of liquid A and B are mixed, then the solution obtained has a volume lesser than 2 litres.

The entropy of mixing is positive as new interactions are introduced into the solution which increases the randomness of the system and hence,  The mixing process is spontaneous and hence

These solutions have vapour pressure greater than that predicted by Raoult’s law.

Examples of solutions showing negative deviation:

1. Acetone + Chloroform

2. Nitric acid HNO3 + water

3. Acetic acid + pyridine

4. Phenol + Aniline

Solutions showing positive deviation from Raoult’s law

These solutions have vapor pressure greater than that predicted by Raoult’s law for the entire range of composition.

This happens when the new interactions are weaker than the interactions in the pure component  (A-B < A-A or B-B interactions). Since the newly formed forces are weaker than the existing forces, heat has to be supplied to break old bonds and form new ones. Hence, the enthalpy change in the mixing of these solutions is positive i.e

The change in the volume during the mixing process is positive i.e. For example if 1 litre solutions each of liquid A and B are mixed, then the solution obtained has a volume greater than 2 litres.

The entropy of mixing is positive as new interactions are introduced into the solution which increases the randomness of the system and hence . The mixing process is spontaneous and hence

These solutions have vapor pressure greater than that predicted by Raoult’s law.
  
 
  

Examples:

1. C₂H₅OH + cyclohexane

2. Acetone + carbon disulphide

3. Acetone + benzene

4. Acetone + Ethyl alcohol

5. Carbon tetrachloride + chloroform or Toluene

6. Methyl alcohol + water

7. Water + Ethyl alcohol