Question

Steam distillation. ANSWER IN DETAIL AND I WILL REWARD FIVE STARS TO FIRST PERSON TO ANSWER!!!!!!

A student steam distilled a mixtrue of toluene (bp 110.8 degrees Celsius) and water and noticed that distillate contained a larger volume of touleune than water. The student said to the professor, "I know there is something wrong with my experiment. I should have more water than toleune because water has a lowerboiling point. Should I redo the experiment?" The professor replied "do whatever you need to do" A classmate then said, "No, you dont need to redo it you DO have more water."

Explain in detail why the classmate is correct. Use calculations to support your explanation. Label all quantities used. Include all units.

Answer 1

An azeotropic mixture has a boiling point that does not change when vapour is removed by evaporation. The boiling point may be higher or lower than any of the components of the mixture.

Toluene / water mixture has bp 85.0oC, but bp toluene is 110.6oC and bp water is 100.0oC. For the toluene-water system, below 85oC the two liquid phases coexist. At 85oC the sum of the vapour pressures of water and toluene = one atmosphere, and boiling starts.

In a mixture of toluene and water, boiling occurs at 85oC and both water and toluene are in the vapour that condenses to form two layers. The bottom layer of water has 0.06% toluene dissolved in it. The top layer of toluene has 0.05% water dissolved in it. The relative volumes are 18% water and 82% toluene. The excess toluene flows back into the flask and distills back over with more water.

At 85oC, vapour presure of water = 57.7 kPA and vapour pressure of toluene = 50.6 kPa. Weight of water = (molar mass water /molar mass toluene) X (vapour pressure water / vapour pressure toluene) = (18 / 92) X (57.7 / 50.6) = 0.223 = 20%.