Homework Help: Chemistry

Posted by Namie on Wednesday, January 4, 2012 at 9:28pm.

When doing a simple distillation of ethanol-water, is the vapor pure ethanol or does it also contain water? (knowing that ethanol has lower boiling point than water)
If it is not pure ethanol, why is it not possible to separate water & ethanol completely?

Thanks in advance.

Chemistry - DrBob222, Wednesday, January 4, 2012 at 10:04pm
Water/ethanol form an azeotrope which is a combination (in this case) of 95.63% ethanol/4.37% H2O and it has a LOWER boiling point (78.2 C) than either H2O (at 100 C) or pure ethanol (78.4 C). Therefore, in a simple distillation, you get the azeotrope(lower boiling point) coming over. Anhydrous ethanol is made by adding benzene, cyclohexane, toluene, (or some others) to the azeotrope and distilling. The added component modifies the boiling point such that pure ethanol can be obtained. Ethanol/water is a combination with a lower boiling point; HCl/H2O form an azeotrope that is a higher boliling point. HCl 20.2%/79.8% H2O boils at 110 C and not -84(HCl) or 100(H2O).

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