This guide covers IGCSE Chemistry topic 4 (part 2): stoichiometry – equations and balancing. You will revise balancing equations, state symbols (s, l, g, aq), reacting mass calculations, and ratio from equation.

Key concepts to revise

• Balancing – Same number of each atom on both sides; use numbers in front of formulae (do not change formulae).
• State symbols – (s) solid, (l) liquid, (g) gas, (aq) aqueous; use when asked.
• Reacting masses – Use equation ratio (moles); moles = mass / Mr; find mass of one substance from mass of another.
• Ratio – From balanced equation (e.g. 2H2 + O2 → 2H2O: 2 mol H2 : 1 mol O2 : 2 mol H2O).

Exam tips and command words

• Balance – Write numbers in front of formulae so atoms balance; check each element.
• State – State symbols for each substance in a given equation.
• Calculate – Mass of product from mass of reactant (mass → moles → ratio → moles → mass).
• Explain – Why the equation must be balanced (conservation of mass; same number of atoms).

Common mistakes

• Changing the formula to balance (e.g. writing H2O2 instead of H2O); only change the number in front.
• Forgetting state symbols when the question asks for them.
• Wrong ratio from equation (e.g. 1 mol A : 2 mol B means 1 mole A reacts with 2 moles B).

Revision checklist

• Balance a given equation and add state symbols.
• Calculate the mass of a product from the mass of a reactant using the equation ratio.
• State what the mole ratio is from a balanced equation.
• Explain conservation of mass in a balanced equation.

Next steps

Book a free trial with an IGCSE Chemistry tutor to practise balancing and reacting mass questions, or explore Tutopiya’s learning portal for more revision resources.