Find the mass of urea needed to prepare 50.1 of a solution in water in which the mole fraction of urea is 7.50×10−2.

Answer 1

50.1 what?

Answer 2

grams.

Answer 3

Let x = mass urea.
n urea = x/60.06
n H2O = 50.1-(x/18)
n urea = [n urea/(n urea + n H2O)]

So (x/60)/[x/60)+{50.1-x}/18] = 0.075 and solve for x.
If I didn't make a math error the answer is approximately 10 g.

Answer 4

where did you get 60.06 from?

Answer 5

oh, molar mass, duh. got it