Posted by Lauren on .
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,
No.
You haven't used Kf, the freezing point constant for water. You just ignored it in the calculation. 
Chemistry 
Mike,
0 is the freezing point of water isnt it

Chemistry 
Lauren,
wait no its 1.858 Kf(degress C/m)

Chemistry 
DrBob222,
right. Kf is 1.86 C/m.

Chemistry 
Lauren,
so it would be 0.085 degrees C = 1.86 C/m (0.1 kg/60 g of (NH2)2CO

Chemistry 
DrBob222,
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,
DrBob222 needs to explain this better