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Cambridge IGCSE Chemistry · 0620

Chapter 3: Stoichiometry — Part 1

Topic 3.1 · Chemical formulae and equations

Formulae and models

Chemical symbols
Each element is represented by a one- or two-letter symbol (e.g. H for hydrogen, Na for sodium, Fe for iron). Symbols are used to write chemical formulae and equations.
Molecular formula
Shows the actual number of atoms of each element in one molecule (e.g. H2O, CO2, C2H6O). It can be deduced from ball-and-stick or dot-and-cross diagrams by counting the atoms shown.
Empirical formula
The simplest whole-number ratio of atoms of each element in a compound (e.g. CH2O for glucose, C6H12O6). For some substances the empirical and molecular formulae are the same (e.g. H2O, CO2).
Ionic compound formulae
Ions have charges. In an ionic compound the total positive charge equals the total negative charge. Write the symbols with charges, then swap and simplify the numbers to give the formula (e.g. Mg2+ and Cl- ? MgCl2; Ca2+ and NO3- ? Ca(NO3)2). Use brackets when a polyatomic ion appears more than once.

Exam Traps

  • Do not write CaNO32 — use brackets: Ca(NO3)2 when the nitrate group appears twice.
  • Do not confuse empirical and molecular formulae — H2O is both for water, but glucose is C6H12O6 (molecular) and CH2O (empirical).

Constructing equations

Word and symbol equations
A word equation names the reactants and products (e.g. magnesium + oxygen ? magnesium oxide). A symbol equation uses chemical formulae and an arrow (?) to show the reaction.
Balancing equations
Atoms are neither created nor destroyed in a chemical reaction. Use coefficients (large numbers in front of formulae) so that the same number of each type of atom appears on both sides. Never change subscripts within a formula.
State symbols
Written in brackets after a formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). Example: 2Mg(s) + O2(g) ? 2MgO(s).
Ionic equations
Show only the ions and molecules that actually take part in the reaction. Spectator ions (unchanged in solution) are omitted from both sides. Example: Ag+(aq) + Cl-(aq) ? AgCl(s).

Exam Traps

  • Do not include unchanged spectator ions (e.g. Na+) in ionic equations for precipitate formation.

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