Raoult’s Law - Practice Questions & MCQ

Negative deviation is seen when total vapour pressure for any mole fraction is less than that expected from Raoult's law. This happens when the new interactions are stronger than the interactions in the pure components (A-B} > A-A or B-B interactions).

• ΔH = -ve, ΔV = -ve
• It forms maximum boiling azeotrope, for example, CHCl₃ + CH₃COCH₃. For such solutions ΔV and ΔH are negative.

Example

1. Chloroform + benzene
2. Acetone + aniline
3. Nitric acid HNO₃ + water
4. Acetic acid + pyridine
5. CH₃OH + Acetone

For such solutions: ΔV and ΔH are negaative.
Here, (V.P)_obs < (VP)_exp
(B.P)_obs > (B.P)_exp
P_A < P_A^oX_A
P_B < P_B^oX_B
P_T < P_A^oX_A + P_B^oX_B

Here ΔS has less value while ΔG has more -ve value and the values of other colligative properties increase.

Positive deviation occurs when total vapour pressure for any mole fraction is more than what is expected according to Raoult's law. This happens when the new interactions are weaker than the interaction in the pure component (A-B < A-A or B-B interactions).

• ΔH = +ve, ΔV = +ve
• It forms minimum boiling azeotropes, for example, C₂H₅OH + cyclohexane. The H-bonding present in pure C₂H₅OH are cut off on adding cyclohexane.
  For such solution, ΔV and ΔH are positive.
  Here, (V.P)_obs > (VP)_exp
  (B.P)_obs < (B.P)_exp
  P_A > P_A^oX_A
  P_B > P_B^oX_B
  P_T > P_A^oX_A + P_B^oX_B
• Here Hydrogen bonds are broken or weakened. Here ΔS is more while ΔG has less -ve value and the values of other colligative properties also decrease.
• Here minimum boiling point azeotrope mixture is formed.

Examples:

1. C₂H₅OH + cyclohexane
2. Acetone + carbon disulphide
3. Acetone + benzene
4. Acetone + Ethylalcohol
5. Carbon tetrachloride + chloroform or Toluene
6. Methylalcohol + water
7. Water + Ethylalcohol