Chapter 12, Problem 1Q

Interpretation Introduction

Interpretation:

The reason for the observed boiling point for the first drop of the collected distillate of pentane and hexane to be higher than the boiling point of pentane should be determined.

Concept introduction:

Distillation is a liquid-gas separation technique for two or more liquids on the basis of their difference in boiling point or vapor pressure.

Vapor pressure is pressure exerted by vapor phase of liquid produced due to an increase in the molecular kinetic energy of the liquid phase or due to the increase in temperature.

The temperature at which atmospheric pressure exerted on liquid equals to its vapor pressure is known to be its boiling point. Vapor pressure of the component decide boiling point of any mixture. Impurities in the mixture might result in an increase or decrease of observed boiling point.

Explanation of Solution

Hexane and pentane are mutually soluble in each other and their solution boil at a temperature intermediate of their boiling point.

In case of pentane and hexane that have $1:1$ molarity, the boiling point that is noticed for the first drop of collected distillate is above boiling point of pentane because pentane has lower boiling temperature than that of hexane. Therefore, when the sample is heated for distillation then pentane will first vaporize, and vapor phase will be rich in pentane so it will exert more vapor pressure initially.

The first drop of the distillate obtained will have a higher boiling point than the pentane as hexane that is present in the mixture will act as impurity and will lead to an increase in its observed boiling point.