As fuels
2 marks · EasyA hydrocarbon burns in exactly 20 g of oxygen during complete combustion. Calculate the mass of carbon dioxide produced if all the oxygen is converted into carbon dioxide. (Assume all atoms from oxygen go into making carbon dioxide; ignore water formation for this calculation. Relative atomic masses: C = 12, O = 16)
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27.5 g (±0.5)
Common mistakes: Not using moles; calculating only O₂ mass as CO₂; forgetting to convert atomic to molecular masses
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Marking points
M1 · 1 mark · Correctly calculates moles of oxygen and sets up correct ratio in the reaction. (follow-through)
A1 · 1 mark · Accurate final mass of carbon dioxide produced. (follow-through)
Worked solution
Step 1: Calculate moles of O₂ used.
Step 2: In complete combustion, every mole of O₂ gives a mole of CO₂ (From equation: )
But oxygen in each CO₂ comes from O₂, and the molar ratio is 1 mole O₂ makes 1 mole CO₂.
So, moles of CO₂ = 0.625
Step 3: Mass of CO₂ = moles × Mr (Mr of CO₂ = 12 + 16 × 2 = 44)
Final answer: 27.5 g