Question

The molarity of urea in a solution prepared by dissolving 16 g of urea (MW = 60.0 g/mol) in 39 g of
H2O is __________ M. The density of the solution is 1.3 g/mL.

i get 8.9 but the practice test says the answer its 6.3

Answer 1

I think the problem is that the SOLUTION has a density of 1.3 g/mL. You are assuming the water is 1.3 g/mL and it isn't.
One way around that is to calculate the molality, then convert that to molarity.

Or you can do it in one step like this.
M = mols/L.
mols = 0.2667
total g soln = 16 from the urea + 39 from water = 55g and volume = 55/1.3 = 42.3 mL or 0.0423 L.
Then 0.2667/0.0423 = 6.3 M.

Answer 2

OH, ok thanks!