Chemistry / Unit 7 Chem 1286 worked example, 60 to 90 seconds
Gas Stoichiometry from Reaction Volumes
The idea
Solving a gas stoichiometry problem means converting a given gas volume to moles with the ideal gas law when conditions are unmatched, applying the balanced equation's mole ratio, then converting back to volume under the target conditions, or using the volume ratio directly when conditions already match.
In practice
Burning propane, C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O, requires O₂ in a 1:5 ratio at matching temperature and pressure, so 2.7 L of propane needs 5 × 2.7 = 13.5 L of O₂ measured under those same conditions, and produces 3 × 2.7 = 8.1 L of CO₂ under the same conditions as well.
Where people slip
Applying the equation's mole-ratio numbers directly to volumes measured at two different temperatures or pressures skips the mole conversion this shortcut depends on.
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