State the formulae of the elements and compounds named in the subject content
Stoichiometry – Formulae (IGCSE Chemistry 0620, Sub‑topic 3.1)
Learning Objective
State the correct chemical formulae (including state symbols) for all elements, simple binary compounds, poly‑atomic ions, common inorganic acids, bases and salts listed in the Cambridge IGCSE Chemistry syllabus. Distinguish between molecular and empirical formulae where required.
1. Elements – Symbols, Atomic Notation & State Symbols (25 °C)
| Element | Symbol | Atomic number (Z) | Mass number (A) – most abundant isotope | State symbol |
|---|---|---|---|---|
| Hydrogen | H | 1 | ¹H | (g) |
| Carbon | C | 6 | ¹²C | (s) |
| Oxygen | O | 8 | ¹⁶O | (g) |
| Nitrogen | N | 7 | ¹⁴N | (g) |
| Sulphur | S | 16 | ³²S | (s) |
| Chlorine | Cl | 17 | ³⁵Cl | (g) |
| Sodium | Na | 11 | ²³Na | (s) |
| Magnesium | Mg | 12 | ²⁴Mg | (s) |
| Calcium | Ca | 20 | ⁴⁰Ca | (s) |
| Potassium | K | 19 | ³⁹K | (s) |
| Iron | Fe | 26 | ⁵⁶Fe | (s) |
2. Molecular vs Empirical Formulae
- Molecular formula – shows the actual number of atoms in a molecule (e.g. C₆H₁₂O₆ for glucose).
- Empirical formula – the simplest whole‑number ratio of atoms (e.g. CH₂O for glucose).
Worked example: Convert the molecular formula C₆H₁₂O₆ to its empirical formula.
- Count the atoms: C = 6, H = 12, O = 6.
- Divide each subscript by the greatest common divisor (6). → C₁H₂O₁.
- Empirical formula = CH₂O.
In the IGCSE syllabus, most compounds are required in their molecular formulae unless the question explicitly asks for the empirical formula (e.g., for ionic compounds that are written in the simplest ratio).
3. Charge‑Balance Rule for Ionic Formulae
Rule: In an ionic compound the total positive charge must equal the total negative charge. Use the charges of the constituent ions to determine the smallest whole‑number subscripts that satisfy this condition.
Example 1 – Monovalent & Divalent ions (Na⁺ & SO₄²⁻):
- Write the ions: Na⁺, SO₄²⁻.
- To balance charges, two Na⁺ (total +2) are needed for each SO₄²⁻ (total –2).
- Formula: Na₂SO₄.
Example 2 – Poly‑atomic ion (NH₄⁺) with a divalent ion (SO₄²⁻):
- Write the ions: NH₄⁺, SO₄²⁻.
- Two NH₄⁺ (total +2) balance one SO₄²⁻ (total –2).
- Formula: (NH₄)₂SO₄.
4. Common Poly‑atomic Ions (required for the syllabus)
| Ion name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | +1 |
| Hydroxide | OH⁻ | –1 |
| Nitrate | NO₃⁻ | –1 |
| Sulphate | SO₄²⁻ | –2 |
| Sulphite | SO₃²⁻ | –2 |
| Carbonate | CO₃²⁻ | –2 |
| Phosphate | PO₄³⁻ | –3 |
| Chlorate | ClO₃⁻ | –1 |
| Acetate | CH₃COO⁻ | –1 |
5. Simple Binary Compounds
5.1 Ionic Binary Compounds (all those listed in the syllabus)
| Name (state) | Formula (state) | Ion charges used |
|---|---|---|
| Sodium chloride (s) | NaCl(s) | Na⁺ + Cl⁻ |
| Sodium oxide (s) | Na₂O(s) | 2 Na⁺ + O²⁻ |
| Potassium chloride (s) | KCl(s) | K⁺ + Cl⁻ |
| Potassium oxide (s) | K₂O(s) | 2 K⁺ + O²⁻ |
| Calcium fluoride (s) | CaF₂(s) | Ca²⁺ + 2 F⁻ |
| Calcium chloride (s) | CaCl₂(s) | Ca²⁺ + 2 Cl⁻ |
| Magnesium chloride (s) | MgCl₂(s) | Mg²⁺ + 2 Cl⁻ |
| Iron(II) chloride (s) | FeCl₂(s) | Fe²⁺ + 2 Cl⁻ |
| Iron(III) chloride (s) | FeCl₃(s) | Fe³⁺ + 3 Cl⁻ |
| Calcium sulphate (s) | CaSO₄(s) | Ca²⁺ + SO₄²⁻ |
| Potassium nitrate (s) | KNO₃(s) | K⁺ + NO₃⁻ |
5.2 Covalent (Molecular) Binary Compounds (syllabus list)
| Name (state) | Formula (state) | Bond type / notes |
|---|---|---|
| Carbon dioxide (g) | CO₂(g) | Two double bonds C=O |
| Carbon monoxide (g) | CO(g) | Triple bond C≡O |
| Water (l) | H₂O(l) | Polar covalent, bent geometry |
| Ammonia (g) | NH₃(g) | Polar covalent, trigonal pyramidal |
| Hydrogen sulphide (g) | H₂S(g) | Polar covalent |
| Hydrogen chloride (g) | HCl(g) | Polar covalent |
6. Inorganic Acids – Formulae (state symbols = aq)
| Acid name | Formula | Acidic ion |
|---|---|---|
| Hydrochloric acid | HCl(aq) | Cl⁻ |
| Sulphuric acid | H₂SO₄(aq) | SO₄²⁻ |
| Nitric acid | HNO₃(aq) | NO₃⁻ |
| Phosphoric acid | H₃PO₄(aq) | PO₄³⁻ |
| Carbonic acid | H₂CO₃(aq) | CO₃²⁻ |
7. Inorganic Bases – Formulae (state symbols = aq)
| Base name | Formula | Notes |
|---|---|---|
| Sodium hydroxide | NaOH(aq) | Strong base |
| Potassium hydroxide | KOH(aq) | Strong base |
| Calcium hydroxide | Ca(OH)₂(aq) | Slaked lime |
| Magnesium hydroxide | Mg(OH)₂(aq) | Milk of magnesia |
| Ammonium hydroxide | NH₄OH(aq) | Weak base (derived from NH₃) |
8. Common Salts – Formulae (state symbols = s)
| Salt name | Formula | Formed from (acid + base) |
|---|---|---|
| Sodium chloride | NaCl(s) | HCl(aq) + NaOH(aq) |
| Calcium sulphate | CaSO₄(s) | H₂SO₄(aq) + Ca(OH)₂(aq) |
| Potassium nitrate | KNO₃(s) | HNO₃(aq) + KOH(aq) |
| Magnesium carbonate | MgCO₃(s) | H₂CO₃(aq) + Mg(OH)₂(aq) |
| Sodium bicarbonate | NaHCO₃(s) | H₂CO₃(aq) + NaOH(aq) |
| Ammonium sulphate | (NH₄)₂SO₄(s) | H₂SO₄(aq) + 2 NH₄OH(aq) |
| Calcium nitrate | Ca(NO₃)₂(s) | 2 HNO₃(aq) + Ca(OH)₂(aq) |
| Potassium chlorate | KClO₃(s) | HClO₃(aq) + KOH(aq) |
| Magnesium phosphate | Mg₃(PO₄)₂(s) | 2 H₃PO₄(aq) + 3 Mg(OH)₂(aq) |
| Iron(II) chloride | FeCl₂(s) | HCl(aq) + Fe(OH)₂(aq) |
9. Practice – Write the Correct Formulae (include state symbols)
- Iron(II) chloride
- Ammonium sulphate
- Calcium nitrate
- Potassium chlorate
- Magnesium phosphate
Answers (teacher reference):
- FeCl₂(s)
- (NH₄)₂SO₄(s)
- Ca(NO₃)₂(s)
- KClO₃(s)
- Mg₃(PO₄)₂(s)
10. Quick Reference Diagram (optional)
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