In an aqueous solution the mole fraction of solute is 0.05. If the solvent is water what is the approximate molality of the solution?
Answer and explanation
Correct answer: 2.92 mol kg−1
Choose a convenient basis of 1 mol total solution. Then solute moles = 0.05 mol and water moles = 0.95 mol. Using the molar mass of water, 18 g mol−1, the solvent mass is 0.95 × 18 = 17.1 g = 0.0171 kg. Molality = moles of solute ÷ kilograms of solvent = 0.05/0.0171 ≈ 2.92 mol kg−1. Hence option B is correct; total solution mass must not be used.
Frequently asked questions
What is the correct answer to this question?
2.92 mol kg−1
Why is this the correct answer?
Choose a convenient basis of 1 mol total solution. Then solute moles = 0.05 mol and water moles = 0.95 mol. Using the molar mass of water, 18 g mol−1, the solvent mass is 0.95 × 18 = 17.1 g = 0.0171 kg. Molality = moles of solute ÷ kilograms of solvent = 0.05/0.0171 ≈ 2.92 mol kg−1. Hence option B is correct; total solution mass must not be used.
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.
