Find the mass of urea (CH4N2O) needed to prepare 50.9 g of a solution in water in which the mole fraction of urea is 7.69×10−2.

Answer 1

Let mass urea = X
mass H2O = 50.9-X

mols urea = X/molar mass urea = ?
mols H2O = (50.9-X)/molar mass H2O = ?
Total mols = X + (50.9-X)/molar mass H2O
Substitute and solve for X. I believe X is approx 11 grams.
Then mols urea/total mols = 0.769