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Statements on Fractional Distillation and Condensation of Vapours

Given below are two statements :

Statement I : Vapours of the liquid with higher boiling point condense before vapours of the liquid with lower boiling points in fractional distillation.

Statement II : The vapours rising up in the fractionating column become richer in high boiling component of the mixture.

In the light of the above statements, choose the correct answer from the options given below :

Options

A

Both Statement I and Statement II are true

B

Both Statement I and Statement II are false

C

Statement I is true but Statement II is false

Correct
D

Statement I is false but Statement II is true

Topics & Concepts

Organic Chemistry - Basic PrinciplesReal Gases

Step-by-Step Solution

To evaluate the correctness of the given statements regarding fractional distillation, let us analyze the physical principles governing the separation technique:

  1. Analysis of Statement I:

    • In fractional distillation, a fractionating column is provided to allow continuous vaporization and condensation cycles. As the mixed vapours move up the column, a temperature gradient is established (hotter at the bottom, cooler at the top).
    • The component with a higher boiling point is less volatile and requires higher kinetic energy to remain in the vapour state. Consequently, as the temperature decreases along the column, the vapours of the higher boiling point liquid condense first (at higher temperatures closer to the base) compared to those of the lower boiling point liquid.
    • Therefore, vapours of the liquid with a higher boiling point condense before vapours of the liquid with a lower boiling point.
    • Thus, Statement I is true.

  2. Analysis of Statement II:

    • According to Raoult's Law and Dalton's Law of partial pressures, the vapour in equilibrium with a liquid mixture is always enriched in the more volatile component (the component with the lower boiling point / higher vapour pressure).
    • As the vapours rise through successive condensation-evaporation stages in the fractionating column, each step further enriches the vapour phase in the lower boiling (more volatile) component, while the liquid returning to the flask becomes richer in the higher boiling component.
    • Therefore, the vapours rising up the fractionating column become richer in the low boiling component, not the high boiling component.
    • Thus, Statement II is false.

Conclusion:

  • Statement I is true but Statement II is false.

This corresponds to Option C.

Statements on Fractional Distillation and Condensation of Vapours | Chemistry PYQ Solution - JEE Challenger