Write ionic formulae and balanced equations
Build neutral ionic formulae from charges and oxidation numbers, then write balanced molecular and ionic equations with suitable states.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the data and constants supplied for the question, show mole-ratio working, and report a justified number of significant figures. Written practice is self-checked
Before you begin
- Use common chemical symbols and formulae, and rearrange a simple equation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must the total charge be in a neutral ionic formula?
What you will learn
- Write ionic formulae from charges, including named ions and oxidation numbers
- Construct balanced equations and net ionic equations with state symbols
Choose a ratio that makes total charge zero
For a neutral ionic compound, combine cations and anions in the smallest whole-number ratio whose total charge is zero. The Periodic Table helps predict many common ion charges; a Roman numeral gives the oxidation number of a metal in the named compound, such as iron(III) for Fe³⁺.
Recall these ions with their charges: NO₃⁻, CO₃²⁻, SO₄²⁻, OH⁻, NH₄⁺, Zn²⁺, Ag⁺, HCO₃⁻ and PO₄³⁻. Use parentheses when more than one polyatomic ion is needed, as in Al₂(SO₄)₃. A formula's subscripts count atoms or polyatomic groups; they do not show charge unless a charge is written.
Balance matter and charge, then remove spectators
A balanced molecular equation has the same number of each atom on each side. Add state symbols when the states are known: (s), (l), (g) and (aq). Coefficients change amounts; never change a formula subscript to balance an equation.
An ionic equation shows the species that change. Split aqueous strong electrolytes into ions, keep solids and liquids intact, and cancel ions that appear unchanged on both sides. The remaining net ionic equation must conserve atoms and total charge; spectator ions are omitted.
Pause and explain
Which is the net ionic equation for precipitation of AgCl?
Worked example
Write the balanced ionic equation, with states, for aqueous silver nitrate reacting with aqueous sodium chloride to form solid silver chloride.
Show the worked solution
- The precipitate is AgCl(s); sodium and nitrate ions remain aqueous.
- Write the full ionic equation with the aqueous salts split into ions.
- Cancel Na⁺ and NO₃⁻, then check atoms and charge.
Answer Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write formulae for iron(III) oxide and ammonium phosphate.
Give me a hint
Balance the total positive and negative charges using the smallest whole-number ratio.
Compare my reasoning
- Fe³⁺ and O²⁻ require total charge +6 and −6.
- NH₄⁺ and PO₄³⁻ require three ammonium ions per phosphate ion.
- Use parentheses around repeated polyatomic ions.
Fe₂O₃ and (NH₄)₃PO₄.
Look for these in your work
- Oxidation number interpreted as Fe³⁺
- Charge neutrality shown
- Parentheses used for repeated ions
Balance the equation for aqueous barium chloride and aqueous sodium sulfate forming solid barium sulfate and aqueous sodium chloride. Then write the net ionic equation.
Give me a hint
Balance the molecular equation first, then cancel unchanged aqueous ions.
Compare my reasoning
- Ba²⁺ combines with SO₄²⁻ in a 1:1 ratio.
- Two NaCl units balance the two chloride ions.
- Na⁺ and Cl⁻ are spectators in the ionic equation.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq); net ionic: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Look for these in your work
- Molecular equation balanced
- State symbols consistent
- Spectators cancelled
A student changes H₂O into H₂O₂ to balance an equation. Explain the correction and give two checks for a valid ionic equation.
Give me a hint
Coefficients change quantities; formulae identify substances.
Compare my reasoning
- Restore the correct water formula.
- Balance using coefficients only.
- Check atom counts and net charge on both sides.
Keep H₂O as the formula and adjust coefficients to balance the equation. A net ionic equation must conserve every atom and total charge, and it must omit spectator ions.
Look for these in your work
- Formula preserved
- Coefficients used
- Atom, charge and spectator checks named
Common mistakes
- Using a subscript to repair an unbalanced equation
- Forgetting parentheses around repeated polyatomic ions
- Keeping spectator ions in a net ionic equation
- Writing an ionic solid as separated aqueous ions
What can you explain now?
What must the total charge be in a neutral ionic formula?
Compare with the explanation
Zero
Cation and anion charges balance to zero.
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Make it stick
How do charge balance, atom balance, state symbols and spectator-ion cancellation apply in that order?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.