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Stoichiometry | IGCSE Chemistry (0620)

Stoichiometry | IGCSE Chemistry (0620) | SM-Educate
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CS-991 · Module I-2b

3 · Stoichiometry

Cambridge IGCSE Chemistry 0620 · Core + Supplement · 2026–2028
Quick Revision
📑 Formulae⚖️ Relative Mass🧬 The Mole

3.1 Formulae Core + Supplement

  • Chemical Formula: Shows the types and numbers of atoms in a substance.
  • Molecular Formula: Actual number and type of atoms in one molecule.
  • Empirical Formula: Simplest whole-number ratio of atoms in a compound.
  • Word Equation: Uses names of reactants and products.
  • Symbol Equation: Uses chemical symbols and formulae.
  • Balanced Equation: Same number of each atom on both sides.
  • State Symbols: (s) Solid • (l) Liquid • (g) Gas • (aq) Aqueous.
  • Ionic Formula: Total positive and negative charges must balance.
  • Ionic Equation: Shows only the ions taking part in the reaction.
📖 Open Full Notes (PDF)
🎥 Video lectures for this sub-topic — coming soon
Q1: The state symbol for a dissolved substance is:
a) (s) b) (l) c) (g) d) (aq)
Show Answer
✅ d) (aq)
Q2: An empirical formula shows:
a) The actual number of atoms in a molecule b) The simplest whole-number ratio of atoms c) Only the ions in a reaction d) The physical state
Show Answer
✅ b) The simplest whole-number ratio of atoms

3.2 Relative Masses of Atoms and Molecules Core

  • Relative Atomic Mass (Ar): Average mass of an atom compared with 1/12 of carbon-12.
  • Relative Molecular Mass (Mr): Sum of the Ar values of all atoms in a molecule.
  • Relative Formula Mass: Sum of atomic masses in an ionic compound.
  • Reacting Masses: Calculated using balanced chemical equations.
  • Conservation of Mass: Total mass of reactants equals total mass of products.
  • Formula: Mr = Sum of Ar values.
📖 Open Full Notes (PDF)
🎥 Video lectures for this sub-topic — coming soon
Q1: Relative atomic mass is measured relative to:
a) Hydrogen-1 b) Oxygen-16 c) 1/12 of Carbon-12 d) Helium-4
Show Answer
✅ c) 1/12 of Carbon-12
Q2: The law of conservation of mass states that:
a) Mass is created in reactions b) Total mass of reactants equals total mass of products c) Mass is destroyed in reactions d) Mass only applies to gases
Show Answer
✅ b) Total mass of reactants equals total mass of products

3.3 The Mole and the Avogadro Constant Core + Supplement

  • 1 Mole: Contains 6.02 × 10²³ particles (Avogadro Constant).
  • Moles Formula: Moles = Mass ÷ Molar Mass.
  • Mass Formula: Mass = Moles × Molar Mass.
  • Particles: Particles = Moles × 6.02 × 10²³.
  • Gas Volume: 1 mole of gas occupies 24 dm³ at r.t.p.
  • Concentration: Concentration = Mass ÷ Volume or Moles ÷ Volume.
  • Molecular Formula = Empirical Formula × Multiplier.
  • Percentage Yield: (Actual Yield ÷ Theoretical Yield) × 100.
  • Percentage Purity: (Pure Substance ÷ Total Sample) × 100.
  • Percentage Composition: (Mass of Element ÷ Mass of Compound) × 100.
📖 Open Full Notes (PDF)
🎥 Video lectures for this sub-topic — coming soon
Q1: One mole of any gas occupies what volume at r.t.p.?
a) 22.4 dm³ b) 24 dm³ c) 100 dm³ d) 1 dm³
Show Answer
✅ b) 24 dm³
Q2: The number of particles in one mole is known as:
a) Molar mass b) Avogadro Constant c) Relative atomic mass d) Percentage yield
Show Answer
✅ b) Avogadro Constant
Q3: Percentage yield is calculated as:
a) Theoretical Yield ÷ Actual Yield × 100 b) Actual Yield ÷ Theoretical Yield × 100 c) Actual Yield × Theoretical Yield d) Actual Yield − Theoretical Yield
Show Answer
✅ b) Actual Yield ÷ Theoretical Yield × 100
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