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