Chemical formulae and equations
Section: 2 Atoms, Molecules and Stoichiometry | Syllabus: Cambridge AS Level Chemistry (9701)
Types of Formula
Empirical Formula The simplest whole-number ratio of atoms of each element in a compound. Molecular Formula The actual number of atoms of each element in one molecule of a compound. It is always a whole-number multiple of the empirical formula.| Compound | Molecular Formula | Empirical Formula |
|---|---|---|
| Water | H₂O | H₂O |
| Hydrogen peroxide | H₂O₂ | HO |
| Glucose | C₆H₁₂O₆ | CH₂O |
| Ethane | C₂H₆ | CH₃ |
| Benzene | C₆H₆ | CH |
Ionic compounds (e.g. NaCl, MgO) are always expressed as empirical formulas - they have no discrete molecules, so a molecular formula is not applicable.
Calculating Empirical and Molecular Formulas
To find the empirical formula from percentage composition or masses:
- Step 1: Divide the mass (or percentage) of each element by its Aᵣ → gives molar ratio.
- Step 2: Divide all values by the smallest value → simplest ratio.
- Step 3: If necessary, multiply to give whole numbers.
A compound contains 40.0% C, 6.7% H, 53.3% O by mass.
Molar ratios: C = 40.0/12 = 3.33 | H = 6.7/1 = 6.7 | O = 53.3/16 = 3.33
Divide by smallest (3.33): C = 1, H = 2, O = 1
Empirical formula: CH₂O
Molecular Formula from Empirical FormulaEmpirical formula CH₂O, Mᵣ = 180
Empirical formula mass = 12 + 2 + 16 = 30
n = 180 ÷ 30 = 6
Molecular formula: C₆H₁₂O₆ (glucose)
Hydrated Salts and Water of Crystallisation
Anhydrous A salt that contains no water of crystallisation. (e.g. anhydrous copper(II) sulfate, CuSO₄) Hydrated A salt that contains water molecules incorporated into its crystal structure. (e.g. hydrated copper(II) sulfate, CuSO₄·5H₂O) Water of Crystallisation Water molecules that are chemically bonded within the crystal lattice of a hydrated salt, shown after a dot in the formula (e.g. CuSO₄·5H₂O - five water molecules per formula unit). Finding the Number of Waters of Crystallisation4.90 g of hydrated barium chloride (BaCl₂·xH₂O) was heated to give 4.17 g of anhydrous BaCl₂.
Mass of water lost = 4.90 − 4.17 = 0.73 g
Moles of BaCl₂ = 4.17 ÷ 208 = 0.0201 mol
Moles of H₂O = 0.73 ÷ 18 = 0.0406 mol
Ratio H₂O : BaCl₂ = 0.0406 ÷ 0.0201 ≈ 2
Formula: BaCl₂·2H₂O
Formulae of Ionic Compounds
Ionic compounds are electrically neutral overall. The formula is determined by balancing the charges of the ions present.
Predicting Ionic Charge from the Periodic Table
- Group 1: 1+ (e.g. Na⁺, K⁺)
- Group 2: 2+ (e.g. Mg²⁺, Ca²⁺)
- Group 13: 3+ (e.g. Al³⁺)
- Group 15: 3− (e.g. N³⁻, P³⁻)
- Group 16: 2− (e.g. O²⁻, S²⁻)
- Group 17: 1− (e.g. Cl⁻, Br⁻)
- Transition metals have variable oxidation states, shown by Roman numerals: Fe²⁺ (iron(II)), Fe³⁺ (iron(III))
Named Polyatomic Ions to Learn
| Ion | Formula | Charge |
|---|---|---|
| Nitrate | NO₃⁻ | 1− |
| Carbonate | CO₃²⁻ | 2− |
| Sulfate | SO₄²⁻ | 2− |
| Hydroxide | OH⁻ | 1− |
| Ammonium | NH₄⁺ | 1+ |
| Zinc | Zn²⁺ | 2+ |
| Silver | Ag⁺ | 1+ |
| Hydrogencarbonate | HCO₃⁻ | 1− |
| Phosphate | PO₄³⁻ | 3− |
Al³⁺ and SO₄²⁻. To balance: 2 × Al³⁺ (total +6) and 3 × SO₄²⁻ (total −6)
Formula: Al₂(SO₄)₃
Writing and Balancing Equations
A balanced chemical equation shows the correct formulae for all reactants and products, with coefficients adjusted so that atoms are conserved.
- Never change a chemical formula to balance - only change the coefficients in front.
- Check that each element has the same number of atoms on both sides.
- Always include state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
Unbalanced: C₃H₈ + O₂ → CO₂ + H₂O
Balanced: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
Ionic Equations
Ionic equations show only the species that actually change during the reaction - spectator ions (ions that appear unchanged on both sides) are omitted.
- Step 1: Write the balanced full equation with state symbols.
- Step 2: Dissociate all soluble ionic compounds (aq) into their ions.
- Step 3: Cancel spectator ions that appear identically on both sides.
- Step 4: Write the net ionic equation.
Full: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Dissociated: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
Cancel spectators (Na⁺, Cl⁻): H⁺(aq) + OH⁻(aq) → H₂O(l)
Ionic Equation: PrecipitationFull: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Ionic: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
(Na⁺ and Cl⁻ are spectator ions and are cancelled)
Exam TipSolids, liquids, and gases are never split into ions in ionic equations - only species labelled (aq) are dissociated. BaSO₄(s) stays as BaSO₄(s).
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