GCSE Chemistry calculations

Practise formula mass, balancing equations, reacting masses, solutions and rates. Choose AQA or Edexcel, then your tier, on the question page.

Before you start

Most calculation mistakes come from one of three things: using the wrong formula, missing a coefficient in an equation or forgetting to convert units. Check those before reaching for the calculator.

Relative formula mass

For MgCl₂, use A_r(Mg) = 24 and A_r(Cl) = 35.5.

M_r = 24 + (2 × 35.5) = 95

Those little subscripts count. A coefficient in front of a formula is a different thing.

Mass from concentration

25.0 cm³ of 0.20 mol/dm³ sodium chloride solution:

25.0 cm³ = 0.0250 dm³

Amount = 0.20 × 0.0250 = 0.0050 mol

Mass = 0.0050 × 58.5 = 0.29 g (2 s.f.)

Choose a calculation

Start with formula mass and balancing equations if you're rusty. Use the later activities for more demanding exam questions.

Relative formula massAdd relative atomic masses, taking care with subscripts and brackets.Balancing equationsUse whole-number coefficients to balance equations without changing the chemical formulae.Reacting massesUse a balanced equation to calculate a mass of reactant or product.Mass from concentrationConvert solution volume when needed, then calculate the mass of solute.Limiting reactantsFind which reactant runs out first. Higher-tier questions also calculate the maximum mass of product.Percentage yield and atom economyCalculate percentage yield from actual and theoretical masses, then work out atom economy.Percentage composition and formulaeCalculate mass percentages and oxygen gained. Edexcel questions also use empirical and molecular formulae.Formulae from percentagesFor Edexcel: calculate formulae from percentages and explain mass conservation in precipitation reactions.Titration readings and calculationsRead burettes and choose concordant titres. Higher-tier questions also calculate concentration.Rates and graphsPlot gas volume against time and calculate mean rate from a table or graph.Rates of reaction: graphsPlot gas-volume data, calculate mean rates and interpret gradients, including tangents where required.Bond energy calculationsUse bond energies to estimate the energy change for a reaction. Higher tier.