Quantitative Chemistry is a core part of GCSE Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Percentage compositionMass of an element ÷ Mr of compound × 100
Theoretical yieldMass of product predicted by the equation
Actual yieldMass of product actually obtained in the lab
Percentage yieldActual yield ÷ theoretical yield × 100
Atom economyMr of useful products ÷ Mr of all reactants × 100
Sustainable reactionHigh yield and high atom economy — less waste
n = m / MrMoles = mass in grams divided by relative formula mass
Mole ratioThe ratio of moles from the balanced equation coefficients
Limiting reactantThe reactant fully used up — it limits the product
Excess reactantA reactant left over after the reaction stops
Empirical formulaSimplest whole-number ratio of atoms in a compound
Molecular formulaActual numbers of each atom in a molecule
MoleAn amount of substance containing 6.02 × 10²³ particles
Avogadro constant6.02 × 10²³ particles per mole
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
Where is almost all the mass of an atom located?✗ Electrons contribute significantly to the mass of an atom. ✓ In the nucleus, made up of protons and neutrons.
What is different between two isotopes of the same element?✗ Isotopes have different numbers of electrons, which is why their mass differs. ✓ They have the same number of protons but different numbers of neutrons.
What does a mole of a substance represent?✗ A mole is a unit of mass equal to the relative molecular mass in grams of any substance. ✓ Avogadro's number (6.02 x 10^23) of particles of that substance.
Why do industries care about atom economy?✗ It makes reactions faster ✓ Higher atom economy means less waste and more sustainability
At room temperature and pressure, what volume does 1 mole of any gas occupy?✗ Heavier gases occupy a smaller volume per mole because their particles are denser. ✓ Approximately 24 dm^3 (24,000 cm^3), regardless of the identity of the gas.
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