Ethanol burns in air
3 marks · NormalCalculate the minimum mass of oxygen needed to completely burn 4.6 g of ethanol ( 2H5OH). (Molar masses: C = 12, H = 1, O = 16)
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9.6 g (±0.2)
Must show: Calculation of moles of ethanol, Mole ratio from equation, Calculation of mass of oxygen
Common mistakes: Mixing up molar masses
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Calculates moles of ethanol correctly: 4.6 / 46 = 0.1 mol.
M2 · 1 mark · Uses mole ratio (1 ethanol : 3 oxygen) to find moles of oxygen needed (0.3 mol). (follow-through)
A1 · 1 mark · Calculates correct mass: 0.3 32 = 9.6 g. (follow-through)
Worked solution
First, write the balanced equation:
Molar mass of ethanol = (2×12) + (6×1) + (1×16) + (1×1) = 24 + 6 + 16 + 1 = 46 g/mol
Moles of ethanol = mass / molar mass = 4.6 g / 46 g/mol = 0.1 mol
From the equation, 1 mol ethanol reacts with 3 mol O2, so 0.1 mol ethanol needs 0.3 mol O2.
Mass of O2 needed = 0.3 mol × 32 g/mol = 9.6 g.