Chemistry / Unit 8 Chem 1568 worked example, 60 to 90 seconds
Calculating Boiling Point Elevation or Freezing Point Depression
The idea
Calculating a boiling point elevation or freezing point depression means multiplying the solute's molal concentration by the appropriate solvent-specific constant, then adding that change to the boiling point or subtracting it from the freezing point of the pure solvent.
In practice
Dissolving 92.1 g of iodine in 800.0 g of chloroform gives a molality of about 0.454 m. Multiplying by chloroform's boiling point elevation constant, 3.63 °C/m, gives a change of about 1.65 °C, raising chloroform's 61.26 °C boiling point to about 62.9 °C.
Where people slip
Adding the calculated change to find a new freezing point, instead of subtracting it, or subtracting to find a new boiling point instead of adding, reverses the direction each colligative effect actually acts in — a mistake easy to make when both constants appear in the same problem.
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Builds on
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