Calculate the mass of Urea required in making 2.5 kg of 0.25 molal aqueous solution.

Molar mass of urea (NH2CONH2) = 2(1 × 14 + 2 × 1) + 1 × 12 + 1 × 16 = 60 g mol−1

0.25 molar aqueous solution of urea means:

1000 g of water contains 0.25 mol = (0.25 × 60)g of urea = 15 g of urea i.e,

(1000 + 15) g of solution contains 15 g of urea

15×2500/1000+15 g

Therefore, 2.5 kg (2500 g) of solution contains = 15×2500/1000+15 = 36.946 g = 37 g of urea (approximately) Hence, mass of urea required = 37 g