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Question 4

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

Answer

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

Therefore, 2.5 kg (2500 g) of solution contains =  (15 X 2500) / (1000+15) g

= 36.95 g

= 37 g of urea (approximately)

Hence, mass of urea required = 37 g

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