IGCSE Chemistry The Mole Notes
The mole lets us count particles by weighing. Every mole calculation uses three ideas: moles ↔ mass, moles ↔ gas volume, and moles ↔ concentration.
Key concepts
- Moles from mass
- n = m ÷ Mᵣ, where m is mass in grams and Mᵣ is relative formula mass.
- Moles from gas volume
- n = V ÷ 24 (V in dm³ at RTP) or n = V ÷ 24 000 (V in cm³).
- Moles from solution
- n = c × V, where c is in mol/dm³ and V in dm³.
- 1 mol of any gas = 24 dm³ at RTP.
- Avogadro's number = 6.02 × 10²³ particles per mole.
- Concentration in g/dm³ = concentration in mol/dm³ × Mᵣ.
Worked example
25.0 cm³ of 0.10 mol/dm³ HCl exactly neutralises NaOH. Calculate the moles of HCl used.
- 1Convert volume to dm³: 25.0 ÷ 1000 = 0.0250 dm³
- 2n = c × V = 0.10 × 0.0250
- 3= 0.00250 mol
Answer: 2.50 × 10⁻³ mol of HCl
Common mistakes
✗ Using cm³ directly in n = c × V.
✓ Always convert cm³ to dm³ first (÷ 1000).
✗ Forgetting the 24 dm³ molar gas volume.
✓ At RTP, 1 mol of any gas = 24 dm³ — learn it.
✗ Confusing Mᵣ with Aᵣ for compounds.
✓ Aᵣ is for atoms; add up Aᵣ of all atoms in the formula to get Mᵣ.
Quick check
Try these, then reveal the answers.
How many moles in 4.0 g of NaOH (Mᵣ = 40)?Reveal answer
0.10 mol
Volume of 0.5 mol of CO₂ at RTP?Reveal answer
12 dm³
Convert 250 cm³ to dm³.Reveal answer
0.250 dm³
Exam tips
- Always write the formula (n = m/Mᵣ, n = V/24, n = cV) before substituting.
- Check units — grams vs kilograms and cm³ vs dm³ trip students up.
- Give answers to 2 or 3 significant figures unless the question states otherwise.
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Frequently asked questions
Why do chemists use the mole?+
Because chemical equations tell us the ratio of particles reacting, and moles let us count particles by weighing manageable masses.
What is molar volume?+
The volume that 1 mol of any gas occupies. At room temperature and pressure it is 24 dm³.
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