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

Answer:

Mass of solution = 2.5 Kg

Number of moles of urea = 0.25 m

Molar mass of urea (NH2CONH2)

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

Molar mass of urea = 60 g mol−1

Mass of urea = Number of moles of urea × Molar mass of urea

Mass of 0.25 moles of urea = 0.25 mol × 60 g mol⁻¹

Mass of 0.25 moles of urea = 15 g

Mass of solution = 1000 g + 15 g = 1015 g

1015 g of aqueous solution contains urea = 15 g

∴ 2500 g of aqueous solution will require urea = 15 g / 1015 kg × 2500 g = 36.95 g = 37 g

Mass of urea required in making 25 grams of 0.25 m = 37 grams

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