Topic 3.3.1: Mole Calculations
Cambridge IGCSE Chemistry 0620 • Paper 4 Style
Exam Instructions:
- Answer ALL questions
- Show ALL working for calculations
- Include units in your final answers
- Use Aᵣ values provided in each question
⚠️ Common Examiner Traps:
- Units Matter: "50" gets 0 marks. Always include units (g, mol, dm³)
- Molar Mass: Must calculate Mᵣ correctly using Aᵣ values given
- Avogadro Constant: Use 6.02 × 10²³, not just "a large number"
- Gas Volume: Use 24 dm³/mol at r.t.p., NOT 22.4 dm³/mol (that's at s.t.p.)
Section A: Definitions and Concepts(5 questions)
Question 1
1 markState the value of the Avogadro constant.
Question 2
2 marksDefine the mole.
Question 3
1 markState the equation that relates moles, mass, and molar mass.
Question 4
1 markWhat is the molar gas volume at room temperature and pressure (r.t.p.)?
Question 5
2 marksExplain why chemists use the mole as a unit rather than counting individual atoms.
Section B: Simple Calculations(5 questions)
Question 6
2 marksCalculate the number of moles in 20 g of calcium. (Aᵣ of Ca = 40)
Question 7
3 marksCalculate the mass of 0.1 mol of sodium carbonate, Na₂CO₃. (Aᵣ: Na = 23, C = 12, O = 16)
Question 8
3 marksHow many moles are in 88 g of carbon dioxide, CO₂? (Aᵣ: C = 12, O = 16)
Question 9
3 marksCalculate the mass of 3 moles of water, H₂O. (Aᵣ: H = 1, O = 16)
Question 10
2 marksWhat volume does 0.5 mol of oxygen gas occupy at room temperature and pressure?
Section C: Particle Counting (Avogadro Constant Applications)(5 questions)
Question 11
2 marksHow many atoms are in 2 moles of helium?
Question 12
2 marksCalculate the number of molecules in 0.25 mol of methane, CH₄.
Question 13
2 marksHow many moles of nitrogen molecules are in 3.01 × 10²³ molecules of N₂?
Question 14
4 marksA sample of aluminum contains 6.02 × 10²⁴ atoms. Calculate: (a) the number of moles of aluminum (b) the mass of the sample (Aᵣ of Al = 27)
Question 15
5 marksA chemist has 96 dm³ of hydrogen gas at r.t.p. Calculate: (a) the number of moles of hydrogen (b) the number of molecules of hydrogen (c) the mass of hydrogen (Aᵣ of H = 1)
🎯 Key Formulas Summary
Moles from Mass:
Moles = Mass (g) ÷ Molar mass (g/mol)
Mass from Moles:
Mass (g) = Moles × Molar mass (g/mol)
Number of Particles:
Particles = Moles × 6.02 × 10²³
Gas Volume (r.t.p.):
Volume (dm³) = Moles × 24
Exam Tip: In Paper 4, examiners award marks for method as well as final answers. Always show your working clearly, even if you make a calculation error. A correct method with a minor arithmetic mistake can still earn most of the marks!
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