A-Level Chemistry · Topic 3
Amount of substance and quantitative chemistry: every key term you need (+ practice quiz)
16 flashcard terms for A-Level Chemistry Topic 3, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
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The mole The amount of substance containing as many particles as there are atoms in 12 grams of carbon-12 — the Avogadro constant.
Empirical formula The simplest whole-number ratio of atoms in a compound. Found by dividing percentage composition by relative atomic masses.
Molecular formula The actual number of each atom in one molecule. Always a whole-number multiple of the empirical formula.
Ideal gas equation Pressure times volume equals moles times the gas constant times absolute temperature. Requires pascals, cubic metres and kelvin.
Molar gas volume One mole of any gas occupies about 24 cubic decimetres at room temperature and pressure, whatever the gas.
Concentration Moles of solute per cubic decimetre of solution. Multiply by volume in cubic decimetres to get moles present.
Percentage yield Actual yield divided by theoretical yield, times one hundred. Losses come from side reactions, transfer and reversibility.
Atom economy The mass of the desired product as a percentage of the total mass of reactants. Addition reactions score one hundred per cent.
Limiting reagent The reactant that runs out first and therefore caps the amount of product. Identify it by dividing moles by stoichiometric coefficient.
Titration Adding a solution of known concentration until the reaction just completes, shown by an indicator colour change at the end point.
Concordant titres Repeat readings within 0.10 cubic centimetres of each other. Only these are averaged, which improves reliability.
Percentage uncertainty Uncertainty divided by the measured value, times one hundred. Using larger volumes reduces it for a fixed apparatus uncertainty.
Water of crystallisation Water molecules built into a crystal lattice. Found by heating to constant mass and comparing the mass lost with the anhydrous salt.
Standard solution A solution of accurately known concentration, made by dissolving a weighed mass and making up to a precise volume in a volumetric flask.
Back titration React a sample with excess reagent, then titrate what remains. Useful when the substance is insoluble or reacts too slowly to titrate directly.
Ionic equation Shows only the species that change, with spectator ions cancelled from both sides. Must balance for both atoms and charge.
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