Reacting masses gases and yields

The mole concept underpins every quantitative calculation in this section.

Mass converts to moles using n = m/M (mass ÷ molar mass), and gas volume converts to moles using n = V/24, with V in dm³ at RTP (or n = V/24000 with V in cm³), or n = pV/RT under any conditions of temperature and pressure. Once the moles of one species are known, the balanced equation provides the ratio needed to convert to moles of any other species, after which conversion back to mass or volume completes the calculation.

Empirical formulae

These are found by converting masses or percentage compositions into moles, dividing each value by the smallest number of moles, and rounding to whole numbers to give the simplest whole-number ratio of atoms in the compound.

Key Definition The molecular formula is a whole-number multiple of the empirical formula, found using the relative molecular mass.

Percentage yield compares the actual mass of product obtained to the theoretical mass and reflects practical losses, while atom economy is calculated only from the balanced equation and reflects the efficiency of converting reactant atoms into the desired product. The two are independent: a reaction can have a high yield with a low atom economy, or vice versa.

Connections across the topic centre on data evaluation. Core Practical 1 measures the molar volume of a gas by reacting magnesium with hydrochloric acid and collecting the hydrogen over water, with deviations from 24.0 dm³ mol⁻¹ explained by gas dissolving in the water or escaping before collection. The same skills appear in determining the water of crystallisation in a hydrated salt and in confirming combustion equations.

Ionic equations distil reactions down to the species that actually change, with state symbols linking directly to test-tube observations:

• Insoluble products form precipitates, shown with the state symbol (s)
• Gases are released as bubbles (effervescence), shown with the state symbol (g)
• Soluble ionic products dissolve to give aqueous ions, shown with the state symbol (aq)

Examiners reward precise observations matched to the specific products identified in the ionic equation.