You have been given the task of purifying a 60 mL sample of toluene. In order to

Question

You have been given the task of purifying a 60 mL sample of toluene. In order to develop your plan of action, you ask about the impurities likely to be present in the sample. You are told that it contains small amounts of acetone (~10%).

For this purification process, you should use fractional distillation because of the small difference in boiling points of the two liquids you are trying to separate (less than 75°C).

Clearly decribe the procedure you would use?

What simple chemical test, performed with the original sample and the purified product, will help you confirm that the acetone impurity has been removed during the purification process?

Explanation / Answer

Fractional distillation is essentially the same as simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with glass or plastic beads. These beads improve the separation between the liquids being distilled. The reason that fractional distillation gives better separation between the liquids is because the glass beads in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compound over and over. The more volatile liquids will tend to push towards the top of the fractionating column, while lower boiling liquids will stay towards the bottom, giving a better separation between the liquids. Of course, the more theoretical plates that you add to a column (the more surfaces or beads), the longer the distillation will take (typically), and the more energy required to keep reevaporating liquid in the fractionating column (this is more of a concern in industrial distillations than in an academic lab where energy cost is not a major cause for worry).

We would use crystallisation to ensure that the impurity has been removed.

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