First find the amount of grams that are in one mole of urea.. you can do this by adding up the atomic masses in the formula. Then do the following calculation where X = the number you just found...
(1 mole urea / X grams urea) (2.57 g urea / 226.5 g water) (1000 g water / 1 kg water)
Place all those side by side, and you will see that the units cancel out give you the molality of the solution (which should be in units of mol/kg)
