Question

1.) You are to perform a distillation of a toluene (BP 110°C at 760 torr) Your water aspirator pulls at 20 torr, What boiling point would you expect for toluene if you distilled using your aspirator? Use the nomograph to show your work?

2.)  When should you choose simple distillation over fractional distillation? 3.)

You’re running a fractional distillation of benzene (80.1°C) from methylbenzene (110.6°C) and you noticed a significant drop in temperature. What does this mean?

Answer 1

1. This is the picture of the pressure-temperature nomograph available in Sigma-Aldrich's website. This interactive nomograph is based on the variation of temperature with pressure. Though the pressure is given in terms of mmHg in the nomograph, as one 1torr ~ 1mmHg, this nomograph can be used here.The boiling point of a liquid with 110^oC at 760torr at 20torr is found to be 13.1^oC. In the nomograph, by setting the boiling point at 1atm (middle column) to be 110^oC and the ambient pressure (third column on the right) to be 20mmHg, the temperature required to achieve the given boiling point at the mentioned pressure is given in the leftmost column as 13.1^oC.

2. When distilling two liquids that are miscible and/or have the tendency to form azeotropes at the temperature applied for distillation with the present concentration, fractional distillation is used to separate the two fluids. Especially, when the difference between the two liquids' boiling points is less than 25^oC, fractional distillation is preferred and when the difference is over 25^oC, simple distillation will suffice.

During the distillation of benzene and toluene, if a large drop in temperature is observed, it signifies the evaporation of the lower boiling liquid, here benzene, while the higher boiling liquid is still present as the higher boiling liquid, toluene has not yet gained enough energy to evaporate, thereby leaving the distillation head bereft of hot vapors.