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Saturday, May 9, 2015

Freezing Point Depression Problems, 2

Category: Chemical Engineering Math

"Published in Newark, California, USA"

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