11  Simple Distillation  Changing Vapor Composition
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11 Simple Distillation Changing Vapor Composition

August 25, 2019


The boiling liquid mixture produces a vapor
mixture. Even though the boiling liquid is a 1:1 mixture of A and B, the composition
of the vapor mixture will initially have more A, since A (with a lower boiling point) has
a higher vapor pressure. In a Simple distillation, the distillate collected at the beginning
will be a mixture of A and B which is mostly A. As A (bp of 80 。) “boils off” more rapidly,
the liquid becomes enriched in B (higher mole fraction, XB). If we periodically change the
receiving flask, we can collect several fractions. Each successive fraction will become richer
in B (bp of 100 。), but will always still contain some A. If we were observing the temperature
of the vapors, we would see it continuously rising as the distillation proceeds to reflect
the increasing content of the higher-boiling B component. Since no pure distillate could
ever be obtained, we can see that a Simple distillation would not be effective at separating
a mixture of liquids.

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