CBSE Class 12-science Answered
how are colligative properties used to measure molecular weight?
Asked by | 13 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/Kf = 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
DTf = 5.48°C - 4.92°C = 0.56°C
m = DTf/Kf = 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 | 14 Jan, 2012, 01:39: PM
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