Percentage yield and atom economyEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student heats 2.43 g of magnesium ribbon in a crucible with a loose lid, lifting the lid from time to time, until the reaction seems to have stopped: 2Mg + O₂ → 2MgO. The white solid left in the crucible has a mass of 3.60 g. Relative atomic masses: O = 16.0, Mg = 24.3.

   (a)

   What is the theoretical yield of magnesium oxide?

   [1 mark]

   • A2.43 g
   • B4.03 g
   • C8.06 g
   • D3.60 g

   (b)

   What is the percentage yield of magnesium oxide?

   [1 mark]

   • A112%
   • B44.7%
   • C89.3%
   • D148%

   (c)

   Use the student's masses to calculate the ratio of the amount of oxygen to the amount of magnesium in the product, and suggest why it differs from the ratio in MgO.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydrogen can be manufactured by steam reforming of methane: CH₄(g) + H₂O(g) → CO(g) + 3H₂(g). Relative atomic masses: H = 1.0, C = 12.0, O = 16.0. Molar masses: CH₄ = 16.0, H₂O = 18.0, CO = 28.0, H₂ = 2.0 g mol⁻¹. Another route to hydrogen is the electrolysis of water: 2H₂O(l) → 2H₂(g) + O₂(g).

   (a)

   What is the percentage atom economy for making hydrogen by steam reforming of methane?

   [1 mark]

   • A82.4%
   • B100%
   • C5.9%
   • D17.6%

   (b)

   Which statement about percentage yield and atom economy is correct?

   [1 mark]

   • AA reaction can have a percentage yield of 100% but a low atom economy
   • BA reaction with a percentage yield of 100% must have an atom economy of 100%
   • CAtom economy is calculated from the mass of product actually collected
   • DAtom economy increases if a catalyst is used

   (c)

   Calculate the percentage atom economy of making hydrogen by the electrolysis of water, and state how it compares with steam reforming.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A company compares two ways of making ethanol, C₂H₅OH (molar mass 46.0 g mol⁻¹). Method 1 is the hydration of ethene. Method 2 is the fermentation of glucose, C₆H₁₂O₆ (molar mass 180.0 g mol⁻¹): C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g). In one batch of method 2, fermentation of 450 kg of glucose gives 168 kg of ethanol.

   (a)

   Calculate the percentage atom economy of method 2 for making ethanol.

   [3 marks]

   (b)

   Calculate the percentage yield of ethanol in the batch.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pharmaceutical company is choosing between two routes, A and B, to make a drug intermediate. Route A has an atom economy of 87% and a percentage yield of 45%. Route B has an atom economy of 62% and a percentage yield of 91%. In the teaching laboratory, students also make aspirin: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. Molar masses: salicylic acid C₇H₆O₃ = 138.0, ethanoic anhydride (CH₃CO)₂O = 102.0, aspirin C₉H₈O₄ = 180.0, ethanoic acid CH₃COOH = 60.0 g mol⁻¹.

   (a)

   Evaluate which route, A or B, is the better choice for the company, using the data on atom economy and percentage yield.

   [6 marks]

   (b)

   A student mixes 5.00 g of salicylic acid with an excess of ethanoic anhydride and obtains 4.86 g of aspirin. Calculate the percentage yield and the percentage atom economy for aspirin, and use your answers to comment on the waste produced.

   [6 marks]

   Total for question 4: 12 marks

End of questions