Empirical Formulae & Formulae of Ionic Compounds
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
What is an Empirical Formula?
The empirical formula is the simplest whole number ratio of atoms of each element in a compound.
| Compound | Molecular Formula | Empirical Formula |
|---|---|---|
| Water | H₂O | H₂O (already simplest) |
| Glucose | C₆H₁₂O₆ | CH₂O (÷ by 6) |
| Ethane | C₂H₆ | CH₃ (÷ by 2) |
| Hydrogen peroxide | H₂O₂ | HO (÷ by 2) |
Sometimes the empirical and molecular formula are the same, if the molecular formula is already in its simplest ratio (as with water above). Calculating an empirical formula from experimental mass or percentage data, and finding a molecular formula from an empirical formula, are covered in detail later in this section, under Empirical & Molecular Formula.
Worked Example: Recognising Empirical Formula
- Question: a compound has molecular formula C₄H₁₀. State its empirical formula. [1]
- Answer: the highest common factor of 4 and 10 is 2; dividing both subscripts by 2 gives C₂H₅ [1]
Formulae of Ionic Compounds
The formula of an ionic compound shows the simplest ratio of ions present. The total positive charge must always equal the total negative charge.
Common Ion Charges
| +1 Ions | +2 Ions | +3 Ions | -1 Ions | -2 Ions |
|---|---|---|---|---|
| Na⁺ (sodium) | Mg₂⁺ (magnesium) | Al³⁺ (aluminium) | Cl⁻ (chloride) | O²⁻ (oxide) |
| K⁺ (potassium) | Ca²⁺ (calcium) | Fe³⁺ (iron(III)) | Br⁻ (bromide) | S²⁻ (sulfide) |
| H⁺ (hydrogen) | Cu²⁺ (copper(II)) | I⁻ (iodide) | ||
| Ag⁺ (silver) | Zn²⁺ (zinc) | OH⁻ (hydroxide) | ||
| NH₄⁺ (ammonium) | Fe²⁺ (iron(II)) | NO₃⁻ (nitrate) |
Polyatomic (multi-atom) ions to know: NH₄⁺ ammonium, OH⁻ hydroxide, NO₃⁻ nitrate, CO₃²⁻ carbonate, SO₄²⁻ sulfate.
An ionic formula shows the simplest ratio of ions in the giant lattice, not a single discrete unit
Deducing Ionic Formulae from Charges
Method: balance the charges. Total positive charge = total negative charge.
- Na⁺ and Cl⁻: already +1/-1 balanced → NaCl
- Mg²⁺ and O²⁻: already +2/-2 balanced → MgO
- Mg²⁺ and Cl⁻: need 2 Cl⁻ to balance +2 → MgCl₂
- Al³⁺ and O²⁻: need 2 Al³⁺ (+6) and 3 O²⁻ (-6) → Al₂O₃
Quick method: "swap and drop". Swap the number from each ion's charge (ignore the +/- sign) and drop it in as the other ion's subscript, then simplify.
- Al³⁺ and O²⁻: swap 3 and 2 → Al₂O₃
- Ca²⁺ and Cl⁻: swap 2 and 1 → Ca₁Cl₂ = CaCl₂ (don't write a subscript 1)
- Mg²⁺ and O²⁻: swap 2 and 2 gives Mg₂O₂, which simplifies to MgO
Using Brackets for Polyatomic Ions
When a polyatomic ion appears more than once in a formula, put it in brackets before adding the subscript.
- Ca²⁺ and OH⁻: need 2 OH⁻ to balance +2 → Ca(OH)₂ (not CaOH₂, which would be a different, wrong formula)
- Al³⁺ and SO₄²⁻: swap and drop gives Al gets 2, SO₄ gets 3 → Al₂(SO₄)₃
- Only use brackets when the polyatomic ion appears 2 or more times - NaOH needs no brackets, but Ca(OH)₂ does
Worked Example: Ionic Formula from Ions
- Question: a fertiliser compound contains only the ions NH₄⁺ and NO₃⁻. State the name and formula of this compound. [2]
- Answer: charges already balanced (+1/-1) so no brackets needed; formula = NH₄NO₃ [1]; name = ammonium nitrate [1]
Practice Examples
| Ions | Formula | Name |
|---|---|---|
| Na⁺ and O²⁻ | Na₂O | Sodium oxide |
| Fe³⁺ and O²⁻ | Fe₂O₃ | Iron(III) oxide |
| NH₄⁺ and Cl⁻ | NH₄Cl | Ammonium chloride |
| Mg²⁺ and NO₃⁻ | Mg(NO₃)₂ | Magnesium nitrate |
| K⁺ and CO₃²⁻ | K₂CO₃ | Potassium carbonate |
| NH₄⁺ and SO₄²⁻ | (NH₄)₂SO₄ | Ammonium sulfate |
Common Mistakes
- ❌ Writing Ca(OH)₂ as CaOH₂
- ✓ without brackets, CaOH₂ incorrectly means 1 Ca, 1 O, 2 H; brackets are essential to show the OH group is doubled: Ca(OH)₂ means 1 Ca, 2 O, 2 H
- ❌ Thinking ionic formulae show individual molecules
- ✓ ionic compounds don't form molecules - they form giant ionic lattices; the formula shows only the simplest ratio of ions, not a discrete unit
- ❌ Adding brackets even when a polyatomic ion appears only once
- ✓ brackets are only needed when the ion appears 2 or more times - NaOH and K₂CO₃ need no brackets
Interactive revision notes, videos and practice questions load below.