Molar mass of urea (NH_{2}CONH_{2}) = 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 That is

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

15x2500/1000+15g

Therefore, 2.5 kg (2500 g) of solution contains

= 36.95 g = 37 g of urea (approximately) Hence, mass of urea required = 37 g

**Note:** There is a slight variation in this answer and the one given in the NCERT textbook.