12 g urea is dissolved in 200 g water. Molar mass of urea is 60 g per mole. What is the molality?
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Answer and explanation
Correct answer: 1.0 mole per kilogram
Molality uses the moles of solute and the mass of solvent, not the total solution mass. Moles of urea = 12 g ÷ 60 g mol−1 = 0.20 mol. The water mass is 200 g = 0.200 kg. Therefore, m = 0.20 ÷ 0.200 = 1.0 mol kg−1, so option A is correct. The other choices result from using an incorrect mass or calculation.
Frequently asked questions
What is the correct answer to this question?
1.0 mole per kilogram
Why is this the correct answer?
Molality uses the moles of solute and the mass of solvent, not the total solution mass. Moles of urea = 12 g ÷ 60 g mol−1 = 0.20 mol. The water mass is 200 g = 0.200 kg. Therefore, m = 0.20 ÷ 0.200 = 1.0 mol kg−1, so option A is correct. The other choices result from using an incorrect mass or calculation.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Chapter 01: Solutions.
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