Homework Help: Chemistry

Posted by Lauren on Monday, February 11, 2008 at 9:31pm.

Urea (NH2)2CO is dissolved in 100.0 g of water. The solution freezes at -0.085 degrees C. how many grams of urea were dissolved to make this solution.

delta T = Kf*molality
Plug in 0.085 for delta T, I assume you know Kf for water, calculate molality.
Then molality x kg solvent x molar mass = grams.
You have all but molar mass which can be calculated.

0.085 degrees C = 1.86 degrees C/m
divide 1.86 and m = .046

then the 100.0g water goes into kg right and then where do i go from here

Chemistry - DrBob222, Monday, February 11, 2008 at 9:38pm
I would carry the 0.046 to at least three places (I know that is one more place than in the 0.085 number BUT I like to carry one more place and round at the end of the work. However, do as your prof advises you to do). So 0.046 = 0.0457 for now.
molality = mol/kg
0.0457 molal = mols urea/0.1 kg solvent.
mols urea = 0.0457 molal x 0.1 molal = 0.00457.
Then mols = grams/molar mass.
You know mols and you know molar mass. Calculate grams.
Chemistry - Lauren, Monday, February 11, 2008 at 10:04pm
is it 60 g of urea for the molar mass annd then take the 0.00457/60 and get grams ???
Chemistry - DrBob222, Monday, February 11, 2008 at 10:14pm
yes

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