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

Exam questions on Yields, atom economy and experimental errors

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.
   Calculate the percentage atom economy for making hydrogen by the two stages combined, CH₄ + 2H₂O → CO₂ + 4H₂.2 marks
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 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
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.
   Calculate the percentage yield of aspirin.3 marks