Question

Calculate the mass of urea (NH₂CONH₂) required in making 2.5 kg of 0.25 molal aqueous solution. 

Answer 1

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 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.