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