Percentage composition by mass: Class 9 practice
The mass percentage of an element = (mass of that element in one formula unit ÷ molar mass) × 100. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Molecular mass
The idea behind percentage composition by mass
The mass percentage of an element = (mass of that element in one formula unit ÷ molar mass) × 100. All percentages add to 100.
Step-by-step method
- Find the molar mass.
- Multiply each element's atomic mass by its count.
- Divide by molar mass × 100.
Worked example 1 (Basic)
Find the percentage by mass of C in C₆H₁₂O₆.
- Molar mass = 180 g/mol
- C: 12 × 6 = 72
- 40%
- Answer: 40%
Worked example 2 (Standard)
Find the percentage by mass of H in CH₄.
- Molar mass = 16 g/mol
- H: 1 × 4 = 4
- 25%
- Answer: 25%
Worked example 3 (Challenge)
Find the percentage by mass of Mg in MgO.
- Molar mass = 40 g/mol
- Mg: 24 × 1 = 24
- 60%
- Answer: 60%
Common mistakes and misconceptions
- Forgetting subscripts
- Percentages not summing to 100
Practice questions
- (Basic) Find the percentage by mass of H in H₂SO₄.
- (Basic) Find the percentage by mass of N in NH₃.
- (Standard) Find the percentage by mass of O in H₂O.
- (Standard) Find the percentage by mass of S in H₂SO₄.
- (Challenge) Find the percentage by mass of O in H₂SO₄.
- (Challenge) Find the percentage by mass of H in H₂O.
Answer key with full working
- Q1: Molar mass = 98 g/mol → H: 1 × 2 = 2 → 2.04% → Answer: 2.04%
- Q2: Molar mass = 17 g/mol → N: 14 × 1 = 14 → 82.35% → Answer: 82.35%
- Q3: Molar mass = 18 g/mol → O: 16 × 1 = 16 → 88.89% → Answer: 88.89%
- Q4: Molar mass = 98 g/mol → S: 32 × 1 = 32 → 32.65% → Answer: 32.65%
- Q5: Molar mass = 98 g/mol → O: 16 × 4 = 64 → 65.31% → Answer: 65.31%
- Q6: Molar mass = 18 g/mol → H: 1 × 2 = 2 → 11.11% → Answer: 11.11%
Syllabus note
This skill is listed under "Atoms and Molecules" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is percentage composition by mass in?
"Atoms and Molecules", Class 9 Chemistry (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.