Question

One mole of benzene is mixed with one mole of toluene. The vapours above the solution contain:

• A. Equal percentages of benzene and toluene
• B. Higher percentage of benzene
• C. Higher percentage of toluene
• D. Only benzene vapours

Hint:

The vapour phase is richer in the component having lower boiling point and higher vapour pressure.

Answer 1

The Correct Option is B

Concept: The vapour phase is always richer in the more volatile component.

Step 1: Compare boiling points. Boiling point of benzene: \[ 80^\circ C \] Boiling point of toluene: \[ 111^\circ C \] Since benzene has lower boiling point, it is more volatile.

Step 2: Compare vapour pressures. More volatile liquids possess higher vapour pressure. Thus: \[ P^\circ_{benzene}>P^\circ_{toluene} \] According to Raoult's law, the component with higher vapour pressure contributes more to the vapour phase.

Step 3: Conclusion. Hence, vapours above the solution contain a greater proportion of benzene. Therefore, option (2) is correct.