A 4 M sodium hydroxide solution has density 1.16 g mL−1. What is its approximate molality?
Answer and explanation
Correct answer: 4.00 mol kg−1
Molarity refers to moles per litre of solution, while molality refers to moles per kilogram of solvent. Take 1 L solution: it contains 4 mol NaOH, whose mass is 4 × 40 = 160 g. The solution mass is 1.16 × 1000 = 1160 g, so water mass is 1160 − 160 = 1000 g = 1 kg. Thus molality = 4/1 = 4.00 mol kg−1, making option C correct. The density is essential for finding solvent mass.
Frequently asked questions
What is the correct answer to this question?
4.00 mol kg−1
Why is this the correct answer?
Molarity refers to moles per litre of solution, while molality refers to moles per kilogram of solvent. Take 1 L solution: it contains 4 mol NaOH, whose mass is 4 × 40 = 160 g. The solution mass is 1.16 × 1000 = 1160 g, so water mass is 1160 − 160 = 1000 g = 1 kg. Thus molality = 4/1 = 4.00 mol kg−1, making option C correct. The density is essential for finding solvent 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.
