1.5 Chemical Formulae, Equations, CalculationsEdexcel IGCSE Chemistry: Revision notes

A word equation shows the names of reactants and products, e.g. magnesium + oxygen → magnesium oxide.

A balanced chemical equation uses formulae and must have equal numbers of each type of atom on both sides, following the law of conservation of mass. State symbols show the physical state of each substance: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

For unfamiliar reactions, use the given information (word equation, formulae, or reaction description) to construct a balanced equation, checking that atoms of each element balance and adjusting the numbers in front of formulae (not the formulae themselves).

Relative formula mass (Mr) is the sum of the relative atomic masses (Ar) of all atoms in a formula.

The mole (mol) is the unit for amount of substance; one mole of any substance contains the same number of particles (6.02 × 10²³, the Avogadro constant).

Key relationships:

• moles = mass (g) ÷ Mr
• mass (g) = moles × Mr

These calculations link amount of substance, relative atomic mass and relative formula mass, and are the basis for all further quantitative chemistry.

To calculate a reacting mass from a balanced equation:

1. Write the balanced equation.
2. Calculate moles of the known substance: moles = mass ÷ Mr.
3. Use the mole ratio from the balanced equation to find moles of the unknown substance.
4. Convert to mass: mass = moles × Mr.

Percentage yield compares the actual mass of product obtained to the maximum theoretical mass possible:

percentage yield = (actual yield ÷ theoretical yield) × 100

Yield is always less than 100% in practice due to losses during the reaction, purification, or transfer, or incomplete/reversible reactions.

• 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 (a whole-number multiple of the empirical formula)

To calculate empirical formula from experimental data (e.g. masses or percentages of each element):

1. Divide each mass/percentage by the Ar of that element to get moles.
2. Divide all mole values by the smallest value to get a simple ratio.
3. If needed, multiply to reach whole numbers.

Experimentally, the formula of a metal oxide can be found by combustion (heating a known mass of metal in air/oxygen and measuring the mass gained) or reduction (passing a reducing gas over the heated oxide and measuring the mass lost). Formulae of water and hydrated salts can be found similarly by heating to drive off water and measuring the mass change.

Concentration in mol/dm³ relates moles, volume and concentration:

moles = concentration (mol/dm³) × volume (dm³)

Molar gas volume: at room temperature and pressure (rtp), one mole of any gas occupies 24 dm³.

• moles of gas = volume (dm³) ÷ 24
• volume (dm³) = moles × 24

These relationships are combined with balanced equations to calculate volumes or concentrations involved in reactions, including titration-type calculations.