Mole concept numericals: Class 9 practice
One mole contains 6.022 × 10²³ particles (Avogadro's number). Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Atomic masses
- Writing formulae
The idea behind mole concept numericals
One mole contains 6.022 × 10²³ particles (Avogadro's number). Number of moles = given mass ÷ molar mass.
Step-by-step method
- Find molar mass from atomic masses (H 1, C 12, N 14, O 16, Na 23, Cl 35.5, Ca 40).
- n = mass ÷ molar mass.
- Particles = n × 6.022 × 10²³.
Worked example 1 (Basic)
How many moles are in 16 g of methane (CH₄)?
- Molar mass = 16 g/mol
- n = 16 ÷ 16 = 1 mol
- Answer: 1 mol
Worked example 2 (Standard)
What is the mass of 2 mol of oxygen gas (O₂)?
- Molar mass = 32 g/mol
- mass = 2 × 32 = 64 g
- Answer: 64 g
Worked example 3 (Challenge)
How many molecules are present in 18 g of water (H₂O)?
- Molar mass = 18 g/mol
- n = 18 ÷ 18 = 1 mol
- Molecules = 1 × 6.022 × 10²³ = 6.022 × 10²³
- Answer: 6.022 × 10²³ molecules
Common mistakes and misconceptions
- Using atomic mass of one atom instead of the molecule
- Confusing atoms with molecules (O₂ has 2 atoms per molecule)
Practice questions
- (Basic) How many moles are in 132 g of carbon dioxide (CO₂)?
- (Basic) How many moles are in 54 g of water (H₂O)?
- (Standard) What is the mass of 1 mol of methane (CH₄)?
- (Standard) What is the mass of 2 mol of methane (CH₄)?
- (Challenge) How many molecules are present in 64 g of oxygen gas (O₂)?
- (Challenge) How many molecules are present in 34 g of ammonia (NH₃)?
Answer key with full working
- Q1: Molar mass = 44 g/mol → n = 132 ÷ 44 = 3 mol → Answer: 3 mol
- Q2: Molar mass = 18 g/mol → n = 54 ÷ 18 = 3 mol → Answer: 3 mol
- Q3: Molar mass = 16 g/mol → mass = 1 × 16 = 16 g → Answer: 16 g
- Q4: Molar mass = 16 g/mol → mass = 2 × 16 = 32 g → Answer: 32 g
- Q5: Molar mass = 32 g/mol → n = 64 ÷ 32 = 2 mol → Molecules = 2 × 6.022 × 10²³ = 12.044 × 10²³ → Answer: 12.044 × 10²³ molecules
- Q6: Molar mass = 17 g/mol → n = 34 ÷ 17 = 2 mol → Molecules = 2 × 6.022 × 10²³ = 12.044 × 10²³ → Answer: 12.044 × 10²³ molecules
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 mole concept numericals 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.