Ethanol burns in air
2 marks · EasyCalculate the total number of oxygen atoms in the products formed when one molecule of ethanol combusts completely in air. Show your working.
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9 (±0)
Common mistakes: Counting the number of molecules, not atoms; Forgetting to multiply by atoms per molecule
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Marking points
M1 · 1 mark · Identifies correct products and multiplies number of molecules by number of O atoms per product.
A1 · 1 mark · Arrives at correct total (9).
Worked solution
Complete combustion:
Each \ce{CO2} has 2 oxygen atoms. There are 2 \ce{CO2} → 2 × 2 = 4 oxygen atoms
Each \ce{H2O} has 1 oxygen atom. There are 3 \ce{H2O} → 3 × 1 = 3 oxygen atoms
Total from products: 4 + 3 = 7 oxygen atoms. But also, \ce{C2H5OH} itself (on the left) contains 1 oxygen atom, and 3 \ce{O2} brings 6 more oxygen atoms for 7 on the right. But question is for total in products: 4 (\ce{CO2}) + 3 (\ce{H2O}) = 7. But each \ce{CO2} is 2 × 2 = 4 and each \ce{H2O} is 3 × 1 = 3, so 4 + 3 = 7.
But checking: is there an ambiguity?
If the question is 'total O atoms in the products', that means from all molecules formed: \ce{2CO2} (4 O atoms) + \ce{3H2O} (3 O atoms) = 7 O atoms.
Final answer: 7