Moles, mass and number of particles: Class 9 practice
One mole contains 6.022 × 10²³ particles and has a mass equal to the molar mass in grams. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Molecular mass
- Scientific notation
The idea behind moles, mass and number of particles
One mole contains 6.022 × 10²³ particles and has a mass equal to the molar mass in grams. n = mass ÷ molar mass = particles ÷ 6.022 × 10²³.
Step-by-step method
- Find the molar mass.
- n = m/M.
- Particles = n × 6.022 × 10²³ (atoms = molecules × atoms per molecule).
Worked example 1 (Basic)
How many moles are in 180 g of C₆H₁₂O₆?
- M = 180 g/mol
- n = 180/180 = 1 mol
- Answer: 1 mol
Worked example 2 (Standard)
How many molecules are in 146.25 g of NaCl?
- n = 2.5 mol
- N = 2.5 × 6.022 × 10²³ = 1.506e+24
- Answer: 1.506e+24
Worked example 3 (Challenge)
How many atoms in total are in 58.5 g of NaCl?
- n = 1 mol of molecules
- Each molecule has 2 atoms
- Atoms = 1 × 2 × 6.022 × 10²³ = 1.204e+24
- Answer: 1.204e+24
Common mistakes and misconceptions
- Confusing molecules and atoms
- Dividing the wrong way
Practice questions
- (Basic) How many moles are in 117 g of NaCl?
- (Basic) How many moles are in 400 g of CaCO₃?
- (Standard) How many molecules are in 16 g of CH₄?
- (Standard) How many molecules are in 408 g of Al₂O₃?
- (Challenge) How many atoms in total are in 40 g of MgO?
- (Challenge) How many atoms in total are in 160 g of NaOH?
Answer key with full working
- Q1: M = 58.5 g/mol → n = 117/58.5 = 2 mol → Answer: 2 mol
- Q2: M = 100 g/mol → n = 400/100 = 4 mol → Answer: 4 mol
- Q3: n = 1 mol → N = 1 × 6.022 × 10²³ = 6.022e+23 → Answer: 6.022e+23
- Q4: n = 4 mol → N = 4 × 6.022 × 10²³ = 2.409e+24 → Answer: 2.409e+24
- Q5: n = 1 mol of molecules → Each molecule has 2 atoms → Atoms = 1 × 2 × 6.022 × 10²³ = 1.204e+24 → Answer: 1.204e+24
- Q6: n = 4 mol of molecules → Each molecule has 3 atoms → Atoms = 4 × 3 × 6.022 × 10²³ = 7.226e+24 → Answer: 7.226e+24
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 moles, mass and number of particles 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.