Question

If a 50% toluene and 50% cyclohexane mixture goes through to vaporization-condensation processes, what is the composition now?

based on a distillation lab

Answer 1

Boiling point (BP) cyclohexane 81ºC

Boiling point (BP) toluene 111ºC

At any given temperature, the vapor pressure of the lower-boiling (lower BP) compound is greater than the vapor pressure of the higher-boiling (higher BP) compound. Thus, the vapor above the liquid will be richer in the lower-boiling compound, compared to the relative amounts in the liquid phase. If we were to collect the vapor, condense it to liquid, and analyze its composition we would find that the vapor was greater than 50% cyclohexane and less than 50% toluene. The vapor is enriched in the lower-boiling cyclohexane.

On doing the experiment, one finds that by carrying out one vaporization - condensation step, starting with a 1:1 mixture of cyclohexane and toluene (50%/50%), the distillate would initially distill as a mixture of 80% cyclohexane and 20% toluene. The distillate has been significantly enriched in cyclohexane. Generally though this would not be considered to be sufficiently pure.

If you take the distillate with 80% cyclohexane and 20% toluene, and distilled again you would find that the distillate is now 95% cyclohexane and 5% toluene. If you take that distillate (95%/5%) and distill it again. This third distillation would produce distillate that is about 99% cyclohexane. This is in the vapor condensed not in the initial mixture.

At the same time, as you remove cyclohexane from the mixture, the residue (liquid in the flask) has been enriched in toluene. By doing three vaporization-condensation steps you have achieved 99% purity in toluene.