a. A mixture of 60 mol % n-proplycyclohexane and 40 mol % n-propylbenzene is dis

Question

a. A mixture of 60 mol % n-proplycyclohexane and 40 mol % n-propylbenzene is distilled through a simple distillation apparatus; assume that no fractionation occurs during the distillation. The boiling temperature is found to be 157 °C (760 torr) as the first small amount of distillate is collected. The standard vapor pressures of n-propylcyclohexane and n-propylbenzene are known to be 769 torr and 725 torr, respectively, at 157.3 °C. Calculate the percentage of each of the two components in the first few drops of distillate.

c. The normal boiling points of n-propylcyclohexane and n-propylbenzene are 156 °C and 159 °C, respectively. Compare the distillation results in Parts a and b. Which of the two mixtures would require the more efficient fractional distillation column for separation of the components? Why?

Explanation / Answer

The percentage composition can be calculated using the vapour pressure data at a give temperature

At 157.3 °C the standard vapor pressures of n-propylcyclohexane and n-propylbenzene are known to be 769 torr and 725 torr.

Since the ratio of the two are 60 mol % n-proplycyclohexane and 40 mol % n-propylbenzene

769 x 0.6 = 461.4 torr

725 x 0.4 = 290 torr

Ptotal = 461.4 + 290 = 751.4 torr

mole fraction of n-proplycyclohexane is 461.4/751.4 x 100 = 61.4 %

mole fraction of n-propylbenzene is 290/751.4 = 38.6%

2) Since the temperature scale is not labelled I will say when the temperature corresponds to the point denoted by the line joing B and A at that temperature the composition of the first few drops of the distillate will be 40mol% toluene and 60mol% benzene.

3)The boiling points of n-propylcyclohexane and n-propylbenzene are 156 °C and 159 °C, wheras the boiling points of benzene and toluene are 80.1°C and 110.6°C respectively.

Since the boiling point of n-propylcyclohexane and n-propylbenzene are very close they will need more efficient fractional distillation column for separation of the components.

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