Empirical and Molecular Formulae

The empirical formula of a compound is the simplest whole-number ratio of atoms.

The molecular formula is the actual number of atoms of each element in one molecule.

For glucose, the molecular formula is C₆H₁₂O₆, but the empirical formula is CH₂O. The ratio of carbon to hydrogen to oxygen is 1:2:1.

For ethene, molecular formula C₂H₄, empirical formula CH₂.

For methane, both are the same: CH₄.

To work out an empirical formula from percentage composition by mass:

First, assume you have 100 g of the compound, so each percentage becomes a mass in grams.

Second, divide each mass by the element’s relative atomic mass to convert into moles.

Third, divide every value by the smallest, giving a ratio.

Fourth, if the ratio isn’t a set of whole numbers, multiply through to clear any halves or thirds.

To find the molecular formula from the empirical formula, you also need the relative molecular mass.

Divide the Mr by the mass of the empirical formula to find the multiplier, then apply that multiplier to every subscript.

If the empirical formula CH₂O has a mass of 30, and the actual Mr is 180, the multiplier is 6 and the molecular formula is C₆H₁₂O₆.

Balanced Equations and Stoichiometry

A balanced chemical equation describes a reaction in terms of ratios of moles.

Take the Haber process: N₂ + 3H₂ → 2NH₃.

One mole of nitrogen reacts with three moles of hydrogen to produce two moles of ammonia.

Stoichiometry is the technique of using these ratios to calculate the amount of one substance from another.

The general method: convert any known quantity to moles, apply the equation ratio to find the moles of what you want, then convert back into mass, volume, concentration or whatever the question asks for.

Some reactants are present in excess, so they don’t all react.

The reactant that runs out first is called the limiting reactant — and it determines the maximum amount of product that can form.

Always identify the limiting reactant before calculating product quantities.

Ionic equations strip away spectator ions to show only the species that change during a reaction.

For the neutralisation NaOH + HCl → NaCl + H₂O, the ionic equation is just H⁺ + OH⁻ → H₂O.

The Na⁺ and Cl⁻ ions don’t change, so they’re left out.

For hydrated salts like CuSO₄·5H₂O, the formula tells you that each formula unit includes five water molecules of crystallisation. These are counted in the relative formula mass.

On heating, the water can be driven off to leave anhydrous CuSO₄.