1)A student carried out a simple distillation on a compound known to distill at
ID: 895212 • Letter: 1
Question
1)A student carried out a simple distillation on a compound known to distill at 124°C and reported a boiling point range of 116-117°C. Gas chromatographic analysis of the product showed the product was pure, and a calibration of thermometer indicated the thermometer was accurate. What procedural error might the student have made in setting up the distillation apparatus?
2)A mixture of toluene, benzene, and 1,2-dimethylbenzene (ortho-xylene) is injected on a gas chromatography column. Predict the sequence of retention times indicating which compound will have the shortest, middle, and longest retention time.
Explanation / Answer
1. The fact that it was pure showed that the student carried out a "good" distillation. That is, it wasn't rushed, but done carefully enough as to separate the components. Thus, the depressed boiling point observed cannot be caused by an impure distillate (the sharp boiling range also indicates purity of distillate). What the student has most likely done is incorrectly positioned the thermometer. The position of the bulb is critical for obtaining an accurate reading of the temperature of the vapour at the point where the vapour just reaches the side-arm that attaches to the condenser. Not low enough, and you will not be measuring the temperature of a vapour that is in equilibrium with the distilling mixture. That explains the depressed boiling range.
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