Chemical Formulae Exam Questions
Practice IGCSE Chemistry exam questions on chemical formulae. Covers writing formulae from names, using valencies, balancing equations, and interpreting formulae of ionic and covalent compounds with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover all aspects of chemical formulae tested on the 0620 papers. Write full answers before checking.
Question 1 (4 marks, Core)
Write the chemical formula for each of the following compounds:
(a) Magnesium oxide (1)
(b) Calcium chloride (1)
(c) Aluminium oxide (1)
(d) Sodium hydroxide (1)
Mark scheme
(a) MgO [1]
(b) CaCl2 [1]
(c) Al2O3 [1]
(d) NaOH [1]
Examiner note: For CaCl2, remember Ca is 2+ and Cl is 1-, so you need two Cl for every Ca. For Al2O3, Al is 3+ and O is 2-, so the ratio is 2:3 to balance charges. Cross-over the valencies to get the subscripts.
Question 2 (3 marks, Core)
Balance the following equations:
(a) Mg + O2 → MgO (1)
(b) Fe + HCl → FeCl2 + H2 (1)
(c) Al + H2SO4 → Al2(SO4)3 + H2 (1)
Mark scheme
(a) 2Mg + O2 → 2MgO [1]
(b) Fe + 2HCl → FeCl2 + H2 [1]
(c) 2Al + 3H2SO4 → Al2(SO4)3 + 3H2 [1]
Examiner note: Never change a formula to balance an equation — only change the numbers in front (coefficients). For (c), work out the aluminium first (2Al gives Al2), then balance sulfate (3 sulfates need 3H2SO4), then check hydrogen (3H2SO4 gives 3H2).
Question 3 (3 marks, Core)
Balance the following equation and add state symbols:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Mark scheme
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g) [1 for formulae, 1 for balancing, 1 for state symbols]
Examiner note: State symbols are (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). CaCO3 is an insoluble solid, HCl and CaCl2 are aqueous, water is liquid, and CO2 is gas. Many candidates forget state symbols when asked.
Question 4 (2 marks, Core)
The formula of iron(III) sulfate is Fe2(SO4)3.
(a) Name the elements present in iron(III) sulfate. (1)
(b) State the total number of atoms represented in one formula unit of Fe2(SO4)3. (1)
Mark scheme
(a) Iron, sulfur, and oxygen [1]
(b) 2 + 3(1+4) = 2 + 3 + 12 = 17 atoms [1]
Examiner note: The bracket means the subscript 3 applies to everything inside: 3 sulfur atoms and 3 x 4 = 12 oxygen atoms, plus 2 iron atoms. Total = 17. A common error is counting SO4 as 5 atoms and forgetting to multiply by 3.
Question 5 (3 marks, Supplement)
Use the valencies given to write the formulae of the following compounds:
Valencies: K = 1+, Fe = 3+, SO4 = 2-, NO3 = 1-, PO4 = 3-
(a) Potassium nitrate (1)
(b) Iron(III) sulfate (1)
(c) Potassium phosphate (1)
Mark scheme
(a) KNO3 [1]
(b) Fe2(SO4)3 [1]
(c) K3PO4 [1]
Examiner note: Cross-over method: for Fe2(SO4)3, Fe is 3+ and SO4 is 2-, cross over to give Fe2(SO4)3. Check: total positive = 2 x 3 = 6+, total negative = 3 x 2 = 6-. They balance.
Question 6 (3 marks, Core)
Balance the following equation for the complete combustion of propane:
C3H8 + O2 → CO2 + H2O
Mark scheme
C3H8 + 5O2 → 3CO2 + 4H2O [1 for CO2 coefficient, 1 for H2O coefficient, 1 for O2 coefficient]
Examiner note: Balance C first (3CO2), then H (8H gives 4H2O), then count O on the right (3x2 + 4x1 = 10 oxygen atoms), so 5O2 on the left. Always balance O last in combustion equations.
Question 7 (2 marks, Core)
A compound has the formula Ca(OH)2.
(a) State the name of this compound. (1)
(b) State the ratio of calcium atoms to oxygen atoms to hydrogen atoms in this compound. (1)
Mark scheme
(a) Calcium hydroxide [1]
(b) 1 : 2 : 2 [1]
Examiner note: The subscript 2 outside the bracket means there are 2 OH groups, so 2 oxygen atoms and 2 hydrogen atoms per formula unit. The ratio is Ca:O:H = 1:2:2.
Question 8 (4 marks, Supplement)
Write balanced symbol equations, including state symbols, for the following reactions:
(a) Zinc reacting with dilute sulfuric acid. (2)
(b) The thermal decomposition of copper(II) carbonate. (2)
Mark scheme
(a) Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) [1 for correct formulae and balancing, 1 for state symbols]
(b) CuCO3(s) → CuO(s) + CO2(g) [1 for correct formulae and balancing, 1 for state symbols]
Examiner note: For (a), zinc is above hydrogen in the reactivity series so it displaces hydrogen. For (b), thermal decomposition of a metal carbonate gives the metal oxide plus carbon dioxide. This pattern applies to most metal carbonates.
What to revise if you scored below 6
If formula writing was difficult (Questions 1, 5), practise the cross-over method with common ions until it is automatic. If balancing equations was the issue (Questions 2, 3, 6), work through 10+ equations, always balancing metals first, then non-metals, then oxygen, then hydrogen. Revisit the chemical formulae notes.
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