Calculations Involving MassesEdexcel GCSE Chemistry: Revision notes

Relative formula mass (Mr) is found by adding together the relative atomic masses (Ar) of all the atoms shown in a formula.

To find the percentage by mass of an element in a compound:

percentage by mass = (Ar × number of atoms of that element ÷ Mr) × 100

An empirical formula shows the simplest whole-number ratio of atoms of each element in a compound. A molecular formula shows the actual number of atoms of each element in a molecule.

To find an empirical formula from reacting masses or percentage composition:

1. Divide each mass (or %) by the element's Ar to get moles
2. Divide each answer by the smallest number of moles
3. Round to whole numbers to get the simplest ratio

To find a molecular formula from an empirical formula: divide the relative molecular mass by the empirical formula mass to get a whole-number multiplier, then multiply each subscript in the empirical formula by it.

An experiment to determine the empirical formula of magnesium oxide involves heating a known mass of magnesium ribbon in a crucible with a lid, allowing air in gradually so all the magnesium reacts with oxygen without escaping as smoke, then reweighing once mass is constant. The mass gained is the mass of oxygen combined.

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction — the total mass of reactants equals the total mass of products.

• In a closed system (e.g. a precipitation reaction in a sealed flask), the total mass measured before and after the reaction stays the same, since no substance can enter or leave.
• In a non-enclosed system (e.g. a reaction in an open flask that gives out or takes in a gas), the measured mass can appear to change because a gas escapes into, or enters from, the surroundings — but the total mass of all substances involved (including the gas) is still conserved.

Using a balanced symbol equation and Ar/Mr values, you can calculate an unknown mass from a known mass:

1. Write the balanced equation
2. Calculate the Mr of the known and unknown substances
3. Use the mole ratio from the equation to scale from known mass to unknown mass

Concentration of a solution is measured in g dm⁻³:

concentration (g dm⁻³) = mass of solute (g) ÷ volume of solution (dm³)

One mole of any substance contains the Avogadro constant number of particles: 6.02 × 10²³ atoms, molecules, formulae or ions. One mole of a substance has a mass equal to its relative particle mass in grams.

number of moles = mass (g) ÷ relative particle mass

In a reaction, the mass of product formed is controlled by the reactant that is not in excess — this reactant is used up first and limits how much product can form (the limiting reactant). By comparing the moles of reactants used and products formed, the stoichiometry (mole ratio) of a reaction can be deduced from experimental masses.