How many moles of solute are required to make a 0.5 molal solution using 250 g water?
Answer and explanation
Correct answer: 0.125 mole
Molality is defined as moles of solute per kilogram of solvent, not per kilogram of solution. The water mass is 250 g = 0.250 kg. Therefore, required solute moles = molality × solvent mass = 0.5 mol kg−1 × 0.250 kg = 0.125 mol. Option A is correct. Using 0.5 directly ignores the 250 g amount, while volume-based reasoning would use the wrong concentration definition.
Frequently asked questions
What is the correct answer to this question?
0.125 mole
Why is this the correct answer?
Molality is defined as moles of solute per kilogram of solvent, not per kilogram of solution. The water mass is 250 g = 0.250 kg. Therefore, required solute moles = molality × solvent mass = 0.5 mol kg−1 × 0.250 kg = 0.125 mol. Option A is correct. Using 0.5 directly ignores the 250 g amount, while volume-based reasoning would use the wrong concentration definition.
Which subject and chapter does this question cover?
This is a Class 11 Chemistry question. Chapter: Chapter 01: Solutions.
