# chemistry

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500g urea aq. Sol. Has mole fraction 0.2 what is molality of sol.

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The mass of the solution is 500. How much is urea and how much is water?
Let X = g urea
and 500-x = grams H2O
Then mols urea = X/60
mol H2O = (500-X/18)

Then
nurea/(nurea + nH2O) = 0.2
(X/60)/{[(X/60)] + [(500-X)/18]} = 0.2

I solved for X and obtained about 227 g for urea and 273 g H2O. You can work that out but that's very close to a mole fraction for urea of 0.2.
Then n urea = about 3.8

From above we had 227 g urea (about 3.8 mols) and 273 g H2O
molality = (mols solute)/(kg solvent)
m = ?
My numbers are estimates; you should recalculate each step.

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