Tue October 16, 2012 By:

1)How are colligative properties used in measuring molecular weight? 2) Why the azeotropes cannot be separated by fractional distillation?

Expert Reply
Tue October 16, 2012
The colligative properties of nonelectrolyte solutions provide a means of determining the molar mass of a solute. Theoretically, any of the four colligative properties are suitable for this purpose. In practice, however, only freezing-point depression and osmotic pressure are used because they show the most pronounced changes. For example, osmotic pressure measurements are very useful for determining the molar masses of large molecules, such as proteins.  

The lowering of the vapor pressure by a solute also brings about other changes: the freezing point is lowered, and the boiling point is raised. The amount of the change, ? TF or ? TB, is determined by the molality (m) of the solution. The relationships are

? TF = KFm
? TB = KFm
Fractional distillation is based on the relative volatility of atleast two components. The relative volatility is a measure of how volatile one component is compared to another at a specified temperature and pressure

But, for some mixtures (e.g. water+ethanol) at a certain temperature and pressure there is no relative volatility (i.e. the volatility of one component is the same as the other component). Such mixtures are known as azeotrope.

The consequence of this azeotrope is that the retrieved fractions from distillation no longer are approximately the pure components but equal to the composition of the binary mixture at boiling point
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