how are colligative properties used to measure molecular weight?

Asked by  | 13th Jan, 2012, 08:55: PM

Expert Answer:

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. An example will help to understand this better.
 
A sample of an unknown organic molecule weighing 1.20 g is dissolved in 50.0 g of benzene.  The resulting solution freezes at 4.92°C.  Determine the molecular weight of the unknown.  Kf = 5.12°C/m and T°f= 5.48°C
      DTf = 5.48°C - 4.92°C = 0.56°C
      m = DTf/K= 0.56°C/5.12°C/m = 0.11 m
      moles of solute = 0.11 m(.0500 kg benzene) = 0.0055 mol solute
      Mw = 1.20 g solute/0.0055 mol solute = 220 g/mol

Answered by  | 14th Jan, 2012, 01:39: PM

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