Homework Help: Chemistry

Posted by Lauren on Saturday, February 9, 2008 at 6:20pm.

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 = 0 degrees C (0.1 kg/60 g of (NH2)2CO

is this right

Chemistry - DrBob222, Saturday, February 9, 2008 at 6:29pm
No.
You haven't used Kf, the freezing point constant for water. You just ignored it in the calculation.
Chemistry - Mike, Saturday, February 9, 2008 at 6:29pm
0 is the freezing point of water isnt it
Chemistry - Lauren, Saturday, February 9, 2008 at 6:30pm
wait no its 1.858 Kf(degress C/m)
Chemistry - DrBob222, Saturday, February 9, 2008 at 7:28pm
right. Kf is 1.86 C/m.
Chemistry - Lauren, Saturday, February 9, 2008 at 7:42pm
so it would be 0.085 degrees C = 1.86 C/m (0.1 kg/60 g of (NH2)2CO
Chemistry - DrBob222, Saturday, February 9, 2008 at 8:48pm
No. You haven't followed the first response I typed.
0.085 = Kf*m
Plug in Kf and solve for m.
Then molality = mols/kg.
You know molality and kg, solve for mols.
Then mols = grams/molar mass. You know mols and you know molar mass, solve for grams.
Chemistry - =Examiner, Sunday, February 28, 2010 at 2:26pm
DrBob222 needs to explain this better

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