Percentage yield and atom economyEdexcel International A Level Chemistry: Flashcards

Exam questions on Percentage yield and atom economy

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.
   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
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).
   Calculate the percentage atom economy of making hydrogen by the electrolysis of water, and state how it compares with steam reforming.2 marks
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.
   Calculate the percentage atom economy of method 2 for making ethanol.3 marks