When 5 g of a nonelectrolyte is dissolved in 250 g of solvent, a 0.2 mol kg⁻¹ solution is formed. What is the molar mass of the substance?
Answer and explanation
Correct answer: 100 g mol⁻¹
Molality is moles of solute divided by kilograms of solvent. Convert 250 g of solvent to 0.250 kg. The moles of solute are therefore n = m × mass of solvent = 0.2 × 0.250 = 0.050 mol. The molar mass is M = mass/n = 5/0.050 = 100 g mol⁻¹. Thus, option B is correct. The mass of solvent, not solution, must be used in the molality formula.
Frequently asked questions
What is the correct answer to this question?
100 g mol⁻¹
Why is this the correct answer?
Molality is moles of solute divided by kilograms of solvent. Convert 250 g of solvent to 0.250 kg. The moles of solute are therefore n = m × mass of solvent = 0.2 × 0.250 = 0.050 mol. The molar mass is M = mass/n = 5/0.050 = 100 g mol⁻¹. Thus, option B is correct. The mass of solvent, not solution, must be used in the molality formula.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Chapter 01: Solutions.
