Comparing the two methods of producing ethanol
3 marks · NormalA chemical company can produce ethanol by either fermentation or hydration of ethene.
The hydration of ethene gives ethanol that is 99% pure by mass. If 5.0 tonnes of ethanol are made, calculate the mass of pure ethanol obtained.
Fermentation produces ethanol that is only 90% pure by mass from the same 5.0 tonnes. Calculate the mass of pure ethanol produced.
Based on your answers, explain one advantage of the hydration method.
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(a) 4.95 tonnes
(b) 4.5 tonnes
(c) Hydration method produces purer ethanol / less energy and processing are required for purification.
Common mistakes: Multiplying by 0.01 instead of correct percentage; Not comparing purities in c)
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Marking points
M1 (a) · 1 mark · Correct calculation: 5.0 × 0.99 = 4.95 tonnes (to 2 d.p.)
M2 (b) · 1 mark · Correct calculation: 5.0 × 0.90 = 4.5 tonnes
B1 (c) · 1 mark · Correct comparison or explanation of advantage (e.g., hydration produces much purer ethanol).
Worked solution
(a) Pure ethanol from hydration = 99% of 5.0 tonnes = 5.0 × 0.99 = 4.95 tonnes
(b) Pure ethanol from fermentation = 90% of 5.0 tonnes = 5.0 × 0.90 = 4.5 tonnes
(c) Hydration gives a higher purity of ethanol, so less purification is needed.