Molecular and formula unit mass: Class 9 practice
Molecular mass is the sum of the atomic masses of all atoms in a molecule. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Chemical formulae
The idea behind molecular and formula unit mass
Molecular mass is the sum of the atomic masses of all atoms in a molecule. Multiply each atomic mass by how many times that atom appears (the subscript), then add.
Step-by-step method
- Write each element with its count.
- Multiply count × atomic mass.
- Add the products; unit u (or g/mol for molar mass).
Worked example 1 (Basic)
Find the molecular mass of C₂H₅OH.
- C: 2 × 12 = 24
- H: 6 × 1 = 6
- O: 1 × 16 = 16
- Total = 46 u
- Answer: 46 u
Worked example 2 (Standard)
Find the molecular mass of MgO.
- Mg: 1 × 24 = 24
- O: 1 × 16 = 16
- Total = 40 u
- Answer: 40 u
Worked example 3 (Challenge)
Find the percentage by mass of C in C₂H₅OH.
- C: 2 × 12 = 24
- H: 6 × 1 = 6
- O: 1 × 16 = 16
- M = 46
- %C = 52.17%
- Answer: 52.17%
Common mistakes and misconceptions
- Forgetting subscripts outside brackets
- Using atomic numbers instead of masses
Practice questions
- (Basic) Find the molecular mass of CO₂.
- (Basic) Find the molecular mass of NaCl.
- (Standard) Find the molecular mass of HNO₃.
- (Standard) Find the molecular mass of NaOH.
- (Challenge) Find the percentage by mass of N in NH₃.
- (Challenge) Find the percentage by mass of Al in Al₂O₃.
Answer key with full working
- Q1: C: 1 × 12 = 12 → O: 2 × 16 = 32 → Total = 44 u → Answer: 44 u
- Q2: Na: 1 × 23 = 23 → Cl: 1 × 35.5 = 35.5 → Total = 58.5 u → Answer: 58.5 u
- Q3: H: 1 × 1 = 1 → N: 1 × 14 = 14 → O: 3 × 16 = 48 → Total = 63 u → Answer: 63 u
- Q4: Na: 1 × 23 = 23 → O: 1 × 16 = 16 → H: 1 × 1 = 1 → Total = 40 u → Answer: 40 u
- Q5: N: 1 × 14 = 14 → H: 3 × 1 = 3 → M = 17 → %N = 82.35% → Answer: 82.35%
- Q6: Al: 2 × 27 = 54 → O: 3 × 16 = 48 → M = 102 → %Al = 52.94% → Answer: 52.94%
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 molecular and formula unit 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.