NCERT Class XII Chemistry
Chapter - Solutions
Questions with Solutions
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Question : 45
Total: 53
Calculate the mass of urea ( N H 2 C O N H 2 ) required in making 2.5 kg of 0.25 molal aqueous solution.
Solution:
Moles of urea = 0.25 m o l e
Mass of solvent (water)= 1 k g = 1000 g
Molar mass of urea( N H 2 C O N H 2 ) = 14 + 2 + 12 + 16 + 14 + 2 = 60 g m o l − 1
∴ Mass of urea in 1000 g of water = 0.25 m o l × 60 g m o l − 1 = 15 g
Total mass of solution= 1000 + 15 g = 1015 g = 1.015 k g
Thus, 1.015 k g of solution contain urea = 15 g
∴ 2.5 k g of solution will require urea =
× 2.5 k g = 37 g
Mass of solvent (water)
Molar mass of urea
Total mass of solution
Thus, 1.015
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