A solution has 20 percent by mass solute and density 1.2 g mL−1. The molar mass of solute is 40. What is the molarity?
Answer and explanation
Correct answer: 6 M
Take exactly 1 L of solution, because molarity is moles of solute per litre of solution. Its mass is density × volume = 1.2 g mL−1 × 1000 mL = 1200 g. A 20% mass fraction gives 0.20 × 1200 = 240 g solute. The solute amount is 240 ÷ 40 = 6 mol, so molarity = 6 mol ÷ 1 L = 6 M. Option D is correct; ignoring density would give an incorrect value.
Frequently asked questions
What is the correct answer to this question?
6 M
Why is this the correct answer?
Take exactly 1 L of solution, because molarity is moles of solute per litre of solution. Its mass is density × volume = 1.2 g mL−1 × 1000 mL = 1200 g. A 20% mass fraction gives 0.20 × 1200 = 240 g solute. The solute amount is 240 ÷ 40 = 6 mol, so molarity = 6 mol ÷ 1 L = 6 M. Option D is correct; ignoring density would give an incorrect value.
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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