AQA GCSE Chemistry (8462) · Paper 1
⚖️ Quantitative Chemistry
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Conservation & Mr
Conservation of mass means that no atoms are created or destroyed in a chemical reaction, they are only rearranged. This means the total mass of the reactants always equals the total mass of the products.
A balanced symbol equation shows the same number of atoms of every element on both sides of the equation. It is balanced by changing only the large numbers in front of each formula, called coefficients, and never the small subscripts within a formula, since changing a subscript would change the substance itself.
Questions about an apparent change in mass usually involve a gas. In an open container, mass can seem to fall because a gas has escaped, for example when a carbonate gives off CO₂, or mass can seem to rise because a gas has been taken in from the air, for example when a metal gains oxygen. In both cases the atoms are all still there; the balance simply cannot detect the gas that has left or joined the reaction.
Moles & concentration
For higher tier, the mole is chemistry's counting unit, equal to 6.02 × 10²³ particles, a number known as the Avogadro constant. One mole of any substance weighs its Mr in grams, so one mole of water weighs 18 g.
For higher tier, reacting mass calculations follow three steps.
- Find the moles of the substance you already know, using moles = mass ÷ Mr.
- Use the ratio of the large numbers in the balanced equation to find the moles of the substance you want.
- Convert those moles back into a mass, using mass = moles × Mr.
For example, in the equation 2Mg + O₂ → 2MgO, 1 mole of Mg produces 1 mole of MgO.
For higher tier, the limiting reactant is the reactant that runs out first, and it decides how much product can form. Every other reactant is described as being in excess, and calculations should be scaled from the limiting reactant.
Yield, economy & titrations Separate science only
Yield is usually below 100% because reactions can be reversible, some product is lost during transfers and separations, and side reactions use up some of the reactants without forming the desired product.
Atom economy measures how much of the mass of the reactants ends up in the desired product. A high atom economy means less waste, so the process is greener and more profitable, and a reaction with only one product has an atom economy of 100%.
For higher tier, concentration in mol/dm³ can be converted to g/dm³ using the Mr: g/dm³ = mol/dm³ × Mr.
For Triple Science, one mole of any gas occupies 24 dm³ at room temperature and pressure.
Titrations (Triple Science)
For higher tier, the calculation for a titration follows the same three-step pattern as reacting masses, and setting out each step clearly, with its units, makes it straightforward to follow.
- Find the moles of the known solution, using moles = concentration × volume, with the volume in dm³.
- Use the ratio in the balanced equation to find the moles of the unknown solution.
- Find the concentration of the unknown, using concentration = moles ÷ volume.