In an aqueous solution the mole fraction of water is 0.90. What is the approximate molality of the solute?
Answer and explanation
Correct answer: 6.17 mol kg−1
The mole fractions of solute and water sum to one. Thus, if xwater = 0.90, xsolute = 0.10. Take 1 mol total mixture: water = 0.90 mol and solute = 0.10 mol. Water mass = 0.90 × 18 = 16.2 g = 0.0162 kg. Molality = 0.10/0.0162 ≈ 6.17 mol kg−1, so option B is correct. The denominator must be solvent mass, not total solution mass.
Frequently asked questions
What is the correct answer to this question?
6.17 mol kg−1
Why is this the correct answer?
The mole fractions of solute and water sum to one. Thus, if xwater = 0.90, xsolute = 0.10. Take 1 mol total mixture: water = 0.90 mol and solute = 0.10 mol. Water mass = 0.90 × 18 = 16.2 g = 0.0162 kg. Molality = 0.10/0.0162 ≈ 6.17 mol kg−1, so option B is correct. The denominator must be solvent mass, not total solution mass.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Some Basic Concepts of Chemistry. Topic: Mole concept and molar mass.
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