Condensation of Benzene from Air

We now consider benzene vapor in air. Air and benzene are very different. At 1 atm pure benzene condenses at 80°C, whereas air condenses at about — 193°C. We will assume that benzene and air form an ideal solution so our data for the vapor pressure of benzene can be used for the partial pressure of benzene in air. We further assume that air is not affected by the presence of a small fraction of benzene. 

Let s quantify our design. Assume that we have approximately 1.3 mol% benzene in air at 1 atm and 26°C. Because benzene and air behave ideally, the partial pressure of benzene is proportional to the mol fraction of benzene  

Whereas pure benzene could be condensed entirely to liquid, it is not possible to condense all the benzene from air by cooling. Even if we cool below the dew point for benzene at 10 torr, some benzene remains in the vapor phase. Consider water vapor in air. A temperature decrease may cause some water vapor to condense (as rain or snow), but not all the water vapor is condensed the relative humidity does not decrease to zero below the dew point. Because some benzene vapor will remain in the air, our destination on the phase map will be on the vapor/liquid border. This is important to remember. 

Thus to condense benzene we must increase its partial pressure to 100 torr, a factor of 10, which means we must increase the total pressure by a factor of 10 from 760 torr to 7,600 torr to condense the first iota of benzene. This is a key concept. 

To condense more benzene, we must increase the pressure further. Recall that pure benzene condensed entirely to liquid when we increased the pressure above the dew point. We crossed into the liquid region on the phase map. With an air-benzene mixture we must remain on the vapor/liquid border. And with the further constraint of constant temperature, we are obliged to remain at the point we first contacted the vapor/liquid border. 

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