The equation : 4NH₃ + 5O₂ ⟶ 4NO + 6H₂O represents the catalytic oxidation of ammonia. If 100 cm³ of ammonia is used calculate the volume of oxygen required to oxidize the ammonia completely.
Topic: Mole concept, gas volumes and stoichiometry
Answer
4NH₃ + 5O₂ ⟶ 4NO + 6H₂O
For gases reacting at the same temperature and pressure, the volumes are in the same ratio as the coefficients (Gay-Lussac's law of combining volumes).
NH₃ : O₂ = 4 : 5
100 cm³ NH₃ needs (5 ÷ 4) × 100 cm³ O₂ = 125 cm³
Volume of oxygen required = 125 cm³
This is the one type of mole-chapter question where you do NOT convert to moles. When everything is a gas at the same temperature and pressure, the balanced equation's coefficients are already the volume ratio — use them directly.
The mistake is inverting the ratio: 4 ÷ 5 × 100 = 80 cm³. Sanity-check it. The equation needs MORE oxygen than ammonia (5 against 4), so the answer must be bigger than 100 cm³. If yours is smaller, you have the fraction upside down.