The mole fraction of solute in water is 0.1. What is the approximate molality of the solution?
Answer and explanation
Correct answer: 6.17 mol kg−1
Assume 1 mol of total solution. A solute mole fraction of 0.1 means 0.1 mol solute and 0.9 mol water. The water mass is 0.9 × 18 = 16.2 g = 0.0162 kg. Molality is moles of solute per kilogram of solvent: 0.1/0.0162 = 6.1728 mol kg−1, approximately 6.17 mol kg−1. Therefore 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?
Assume 1 mol of total solution. A solute mole fraction of 0.1 means 0.1 mol solute and 0.9 mol water. The water mass is 0.9 × 18 = 16.2 g = 0.0162 kg. Molality is moles of solute per kilogram of solvent: 0.1/0.0162 = 6.1728 mol kg−1, approximately 6.17 mol kg−1. Therefore 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.
