Question

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

Answer 1

Given: Moles of urea = 0.25 mol

Mass of solvent (water) = 1 kg = 1000 g

Molar mass of urea (NH₂CONH₂) = 2(1 × 14 + 2 × 1) + 1 × 12 + 1 × 16

= 60 g mol⁻¹

∴ Mass of urea in 1000 g of water = 0.25 mol × 60 g mol⁻¹

= 15 g

Total mass of solution = 1000 + 15 g

= 1015 g

= 1.015 kg

Thus, 1.015 kg of solution contains urea = 15 g

∴ 2.5 kg of solution will require urea = `(15  "g")/(1.015  "kg") xx 2.5  "kg"`

= 36.95 g

= 37 g (approximately)

Hence, the mass of urea required is 37 g.