Cambridge IGCSE Chemistry 3 Stoichiometry Exam Questions

About Cambridge IGCSE Chemistry 3 Stoichiometry

Stoichiometry is the calculations topic of Cambridge IGCSE Chemistry 0620. It begins with writing formulae and balanced equations with state symbols, then moves to relative atomic and molecular masses. Extended candidates go further with the mole and the Avogadro constant, reacting masses, molar gas volume, solution concentration, empirical and molecular formulae, and percentage yield, composition and purity. Because these skills are needed in electrochemistry, acids and salts, titrations and organic chemistry, marks from stoichiometry appear all over the theory papers. Examiners test whether you can lay out multi-step working clearly, use the mole ratio from the equation and convert cm3 to dm3 correctly.

What you need to know

• 3.1 Formulae and equations Deduce formulae from ions or from models and diagrams, and write word and balanced symbol equations with state symbols (s), (l), (g) and (aq). Extended candidates also write ionic equations, leaving out spectator ions.
• 3.2 Relative masses Relative atomic mass, Ar, is the average mass of the isotopes of an element compared with 1/12 of the mass of a carbon-12 atom. Relative molecular (or formula) mass, Mr, is the sum of the Ar values in the formula.
• 3.3 The mole (Supplement) One mole contains 6.02 x 10^23 particles (the Avogadro constant). Number of moles = mass / molar mass. One mole of any gas occupies 24 dm3 at room temperature and pressure.
• 3.3 Reacting masses and gases Convert the known quantity to moles, use the mole ratio from the balanced equation, then convert back to mass or gas volume. Identify the limiting reactant when amounts of two reactants are given.
• 3.3 Concentration Concentration in mol/dm3 = moles / volume in dm3; it can also be expressed in g/dm3. Divide cm3 by 1000 to get dm3. Titration results are used to calculate an unknown concentration.
• 3.3 Formulae from data Empirical formula is the simplest whole number ratio of atoms: divide each mass or percentage by Ar, then by the smallest answer. The molecular formula is a whole number multiple of the empirical formula, found using Mr.
• 3.3 Percentages Percentage yield = actual yield / theoretical yield x 100. Percentage purity = mass of pure product / mass of impure sample x 100. Percentage composition by mass uses the Ar values in the formula.

Common mistakes in 3 Stoichiometry

• Forgetting to use the mole ratio from the equation. Always check whether one mole of reactant gives one, two or half a mole of product.
• Using volume in cm3 in concentration calculations. Divide by 1000 to convert to dm3 first.
• Using 24 dm3 per mole for solids, liquids or solutions. The molar gas volume applies only to gases at room temperature and pressure.
• Rounding at each step. Keep full values in your calculator and only round the final answer, usually to three significant figures.
• Using Ar instead of Mr for diatomic elements, for example taking the molar mass of oxygen gas as 16 g/mol instead of 32 g/mol.

Try three 3 Stoichiometry exam-style questions