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What is the freezing point of a 30% urea, (NH2)2CO solution in water?
Solution:
Basis: 100 grams of 30% (NH2)2CO
The freezing point constant is defined as the number of degrees the freezing point will be lowered per mole of solute per 1000 g or 1 kg of solvent present. This can be written as
where m is the molality of a solution.
The number of moles of (NH2)2CO is
The weight of water is
Hence, the molality of 30% (NH2)2CO solution in water is
From the Table of Freezing and Boiling Information of Solvents, the freezing point constant of water is
.The freezing point depression is defined as the product of the freezing point constant of a solvent and the molality of a solution.
Hence, the freezing point depression of a solution is
Therefore, the freezing point of 30% (NH2)2CO solution is
