Yields, atom economy and experimental errorsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Hydrogen for fuel cells is made industrially by steam reforming of methane. In the first stage, CH₄(g) + H₂O(g) → CO(g) + 3H₂(g). In a second stage the carbon monoxide reacts with more steam: CO(g) + H₂O(g) → CO₂(g) + H₂(g). Use relative formula masses: H₂ 2.0, H₂O 18.0, CH₄ 16.0, CO 28.0, CO₂ 44.0.

   (a)

   What is the percentage atom economy for making hydrogen in the first stage, CH₄ + H₂O → CO + 3H₂?

   [1 mark]

   • A21.4%
   • B82.4%
   • C100%
   • D17.6%

   (b)

   A plant reacts 1000 kg of methane and obtains 150 kg of hydrogen by the first-stage reaction. What is the percentage yield of hydrogen?

   [1 mark]

   • A15.0%
   • B40.0%
   • C80.0%
   • D250%

   (c)

   Calculate the percentage atom economy for making hydrogen by the two stages combined, CH₄ + 2H₂O → CO₂ + 4H₂.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student evaluates the apparatus used in a titration practical. The 50.0 cm³ burette has an uncertainty of ±0.05 cm³ for each reading, the 25.0 cm³ pipette has an uncertainty of ±0.06 cm³, and the balance has an uncertainty of ±0.001 g for each reading.

   (a)

   A titre of 20.00 cm³ is calculated from an initial and a final burette reading. What is the percentage uncertainty in the titre?

   [1 mark]

   • A0.50%
   • B0.25%
   • C0.10%
   • D5.0%

   (b)

   Which change would reduce the percentage uncertainty in a mass measured on this balance?

   [1 mark]

   • AUse a smaller mass of solid
   • BUse a balance that reads to fewer decimal places
   • CUse a larger mass of solid
   • DWeigh the solid in a larger container

   (c)

   A student weighs a sample by difference: the container and solid has a mass of 5.432 g and, after the solid is tipped out, the container has a mass of 4.287 g. Calculate the mass of solid transferred and its percentage uncertainty.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Aspirin, C₉H₈O₄, is made by reacting salicylic acid with ethanoic anhydride: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. A student reacts 5.00 g of salicylic acid with excess ethanoic anhydride and obtains 4.80 g of pure aspirin. Use M(salicylic acid) = 138.0, M(aspirin) = 180.0 and M(ethanoic acid) = 60.0 g mol⁻¹.

   (a)

   Calculate the percentage yield of aspirin.

   [3 marks]

   (b)

   Calculate the percentage atom economy for this preparation of aspirin and the mass of ethanoic acid formed in theory from 5.00 g of salicylic acid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student plans to find the percentage by mass of calcium carbonate in a sample of limestone. She will add about 0.20 g of the limestone, weighed by difference on a balance with an uncertainty of ±0.01 g in each reading, to 50 cm³ of 2.0 mol dm⁻³ hydrochloric acid measured in a 100 cm³ measuring cylinder (±0.5 cm³). The flask is connected to a 100 cm³ gas syringe (±0.5 cm³) to collect the carbon dioxide. Take the molar volume as 24.0 dm³ mol⁻¹.

   (a)

   Evaluate the risks in this procedure and explain the precautions the student should take.

   [6 marks]

   (b)

   Calculate the percentage uncertainty in the measured mass of limestone and in the volume of carbon dioxide, and evaluate how the student could reduce the percentage error in her results.

   [6 marks]

   Total for question 4: 12 marks

End of questions