Question

Describe how you can purify a sample of toluene suspected to be contaminated with benzene?

Answer 1

Toluene and Benzene can be separated from one another by the method of fractional distillation.

Toluene is less volatile with a boiling point of 110.6^oC where as benzene is the more volatile component with a boiling point of 80.6^oC. Snce both these liquids have a small difference in boiling points, they must be separated by fractional distillation.

Instrumentation - Fractional distillation contains the same set up as simple distillation but has an additional fractionating column on the vessel containing the mixture. The column is packed with glass beads that increase the surface area.

Working - The mixture is heated to a high temperature and converted to vapours. These vapours rise in the fractionating column.

The vapours are richer in the more volatile component that is benzene and poorer in the less volatile component that is toluene.

As the vapours rise, they hit the beads and condense. Being more volatile, more and more benzene vapours rise towards the top and condense by hitting the beads. Being less volatile, vapours of toluene reach a lower height and condense towards the bottom of the column.

The process can be repeated a few times till we separate benzene from the top of the column leaving behind pure toluene in the vessel. It can later be removed from the bottom of the vessel.