In a closed system, the total mass of products is
less than the total mass of reactants when a gas is made
equal to the total mass of reactants
greater than the total mass of reactants when a precipitate forms
less than the total mass of reactants because some atoms are destroyed
55 exam-style questions on Edexcel GCSE Chemistry 2.6 Calculations involving masses, covering 2.6.1 Relative formula mass and percentage composition, 2.6.2 Empirical and molecular formulae, 2.6.3 Conservation of mass, 2.6.4 Reacting masses and concentration in g dm⁻³, 2.6.5 The mole and the Avogadro constant, and 2.6.6 Limiting reactants and stoichiometry. Each one has a worked solution and a mark scheme showing where the marks go.