Ethane burns in oxygen to form CO₂ and H₂O according to the equation: 2C₂H₆ + 7O₂ ⟶ 4CO₂ + 6H₂O If 1250 cc of oxygen is burnt with 300 cc of ethane. Calculate:
the volume of unused O₂.
Topic: Organic chemistry: structure, nomenclature and reactions
Answer
From part (i), ethane is the limiting reactant and oxygen is in excess.
Oxygen actually used: C₂H₆ : O₂ = 2 : 7 O₂ used = (7 ÷ 2) × 300 = 1050 cc
Oxygen unused = oxygen supplied − oxygen used = 1250 − 1050 = 200 cc
Volume of unused O₂ = 200 cc
Two subtractions catch people out here. Subtract the oxygen USED from the oxygen SUPPLIED — not from the carbon dioxide formed, and not the other way round.
And note that the leftover oxygen is still in the container at the end. If the question had asked for the composition of the final mixture, the answer would be 600 cc of CO₂ plus 200 cc of unused O₂ — the water is a liquid at room temperature and takes up no measurable gas volume.