AS · Practice questions · Formulae and equations

Every atom accounted for.

Six original Cambridge-style questions on ionic formulae, balanced equations with state symbols and ionic equations, including a reaction with an insoluble solid.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Formula first, balance second.

Ionic formula: total positive charge = total negative charge.
Brackets round a polyatomic ion when there is more than one.
Balance with coefficients only; leave O and H to last in combustion.
Ionic equation: drop spectators; atoms and charge must both balance.

Recall

[4 marks]

Write the formulae of (a) ammonium sulfate, (b) iron(III) hydroxide, (c) sodium hydrogencarbonate, (d) copper(II) nitrate.

(a) NH₄⁺ and SO₄²⁻: (NH₄)₂SO₄. ✓
(b) Fe³⁺ and OH⁻: Fe(OH)₃. ✓
(c) Na⁺ and HCO₃⁻: NaHCO₃. ✓
(d) Cu²⁺ and NO₃⁻: Cu(NO₃)₂. ✓

Calculation

[2 marks]

Write a balanced equation for the complete combustion of butane, C₄H₁₀.

C₄H₁₀ + 6½O₂ → 4CO₂ + 5H₂O (4 C, 10 H, then 8 + 5 = 13 O atoms). ✓
Doubling to clear the fraction: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. ✓

Recall

[2 marks]

Write balanced equations, with state symbols, for (a) the thermal decomposition of calcium carbonate and (b) the reaction of magnesium with dilute hydrochloric acid.

(a) CaCO₃(s) → CaO(s) + CO₂(g). ✓
(b) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). ✓

Recall

[2 marks]

Aqueous silver nitrate is added to aqueous sodium chloride. Write the ionic equation, with state symbols, and name the spectator ions.

Ag⁺(aq) + Cl⁻(aq) → AgCl(s). ✓
The spectator ions are Na⁺ and NO₃⁻. ✓

Analysis

[2 marks]

Write the ionic equation, with state symbols, for the reaction of solid calcium carbonate with dilute hydrochloric acid. Explain why calcium carbonate is not written as separate ions.

CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). Charge: +2 on each side. ✓
Calcium carbonate is an insoluble solid, so its ions are not free in solution; only dissolved ionic compounds are split into ions. ✓

Explain

[2 marks]

A student balances H₂ + O₂ → H₂O by writing H₂ + O₂ → H₂O₂. Explain why this is wrong and give the correct equation.

Changing the subscript changes the substance: H₂O₂ is hydrogen peroxide, not water. Only coefficients may be changed. ✓
2H₂ + O₂ → 2H₂O. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.