Stoichiometry is a core part of IB Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Limiting reagentRuns out first, caps product
Excess reagentLeft over at end
Theoretical yieldMax product from stoichiometry
Percentage yieldactual/theoretical × 100
Atom economyMr(product) / Mr(reactants) × 100
Green chemistryHigh atom economy + safe solvents
Ideal gasNo volume, no IMF — model approximation
pV = nRTp in Pa, V in m³, T in K, R = 8.31
STP100 kPa, 273.15 K (definition)
Molar volume22.7 dm³ mol⁻¹ at STP
Boyle's lawp ∝ 1/V at constant T, n
Charles' lawV ∝ T at constant p, n
Mole6.02×10²³ particles of substance
Avogadro constantNA = 6.02×10²³ mol⁻¹
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
Does one mole of every substance have the same mass?✗ Yes, a mole of anything weighs the same. ✓ No — same number of particles, but mass depends on the formula mass.
What does the big number in front of a formula multiply?✗ Only the first element in the formula. ✓ The whole formula (e.g. 2H₂O = 4 H and 2 O).
What are the units of Kc for a reaction?✗ Kc always has units of mol dm^-3. ✓ They depend on Δn (the difference between moles of gaseous/aqueous products and reactants) — Kc can be dimensionless or have units like mol dm^-3.
What state are the ions in when defining lattice enthalpy of formation?✗ The ions are in aqueous solution before forming the lattice. ✓ Gaseous ions — lattice enthalpy is the energy change when one mole of solid ionic compound forms from its constituent ions in the GAS phase.
What does the M+ peak in a mass spectrum represent?✗ The M+ peak shows the mass of the largest fragment after the molecule breaks apart. ✓ The molecular ion — formed when a single electron is removed from the whole molecule, giving the relative molecular mass.
Practise Stoichiometry — free games
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