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

Given,

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

1000 g of water contains 0.25 mol = (0.25 × 60) g of urea.

= 15 g of urea.

Means, 1015 g of solution contains 15 g of urea

Therefore,

2500 g of solution contains =

= 36.95 g

Hence, mass of urea required is 37 g (approx).