Alcohols
5 marks · EasyEthanol can be used as a fuel. The equation for its complete combustion is:
State the molecular formula of ethanol.
How many moles of carbon dioxide are made from 1 mole of ethanol in complete combustion?
Calculate the total number of oxygen atoms found in the water produced when 0.5 mol of ethanol burns completely. (Show your working.)
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(a) C2H6O · must show Correct molecular formula
(b) 2 mol · must show Correct ratio from equation
(c) 1.5 mol H2O, which contains 3 mol atoms of oxygen. 3 mol × 6.02 × 10^{23} = 1.806 × 10^{24} atoms. OR 3 × 0.5 = 1.5 mol H2O = 3.0 × 0.5 = 1.5 mol O atoms = 9.03 × 10^{23} atoms. · must show Moles of H2O calculated (1.5 mol), Recognizes 1 H2O contains 1 O atom, Converts to number of atoms (1.5 mol × 6.02 × 10^{23})
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Marking points
B1 (a) · 1 mark · States correct molecular formula (C2H6O).
B2 (b) · 1 mark · States 2 moles CO2 produced per mole ethanol, from balanced equation.
B3 (c) · 1 mark · Calculates number of moles of water made from 0.5 mol ethanol (1.5 mol).
B4 (c) · 1 mark · Identifies 1 O atom per H2O molecule and determines moles of O atoms (1.5 mol).
B5 (c) · 1 mark · Converts moles of O atoms to total number of atoms (1.5 × 6.02 × 10^{23} = 9.03 × 10^{23}). Allow low sig fig.
Worked solution
a) The molecular formula of ethanol is C2H6O.
b) From the equation: 1 mole ethanol produces 2 moles CO2.
c) From the equation: 1 mole ethanol produces 3 moles water. 0.5 mol ethanol yields 3 × 0.5 = 1.5 mol H2O. Each water molecule (H2O) contains 1 O atom, so that's 1.5 mol O atoms. Number of atoms = 1.5 × 6.02 × 10^{23} = 9.03 × 10^{23} atoms.