Ethanol burns in air
2 marks · EasyCalculate the mass of carbon dioxide produced when 4.6 g of ethanol burn completely in air.
(Relative atomic masses: C = 12, H = 1, O = 16)
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8.8 g (±0.1)
Common mistakes: Forgets to multiply by 2 for moles of CO2; Uses molar mass of CO2 incorrectly
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Marking points
M1 · 1 mark · Calculates moles of ethanol: (4.6/46), then links to moles of CO2 produced (×2 from equation).
A1 · 1 mark · Correctly calculates mass of CO2 produced (8.8g).
Worked solution
Ethanol formula: C2H5OH (molar mass = (2 × 12) + (6 × 1) + (1 × 16) = 24 + 6 + 16 = 46 g/mol).
Moles of ethanol: 4.6 g ÷ 46 g/mol = 0.1 mol.
From the equation, 1 mol ethanol gives 2 mol CO2. So, 0.1 mol gives 0.2 mol CO2.
Molar mass CO2 = (12 + 16 × 2) = 44 g/mol.
Mass of CO2 = 0.2 mol × 44 g/mol = 8.8 g.