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A second set of questions on percentage yield and atom economy. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
Calculate the atom economy for iron in Fe₂O₃ + 3CO → 2Fe + 3CO₂ to 3 significant figures. Mr of Fe₂O₃ = 160 and CO = 28, Ar of Fe = 56.
Answer: 45.9%
5.0 g of CaCO₃ is heated and the theoretical yield of CaO is 2.8 g. The student collects 2.1 g. Calculate the percentage yield.
Answer: 75%
20.0 g of calcium carbonate is heated: CaCO₃ → CaO + CO₂. Calculate the theoretical yield of calcium oxide. Mr of CaCO₃ = 100 and CaO = 56.
Answer: 11.2 g
Calculate the atom economy for hydrogen in 2H₂O → 2H₂ + O₂ to 2 significant figures. Mr of H₂O = 18 and H₂ = 2.
Answer: 11%
Calculate the theoretical yield of MgO from 4.8 g of Mg in 2Mg + O₂ → 2MgO. Ar of Mg = 24 and Mr of MgO = 40.
Answer: 8.0 g
A student collects 6.4 g of magnesium oxide. The theoretical yield is 8.0 g. Calculate the percentage yield.
Answer: 80%
Calculate the atom economy for making sodium hydroxide in 2Na + 2H₂O → 2NaOH + H₂ to 3 significant figures. Mr of Na = 23, H₂O = 18, NaOH = 40.
Answer: 97.6%
Explain why a reaction with a high percentage yield may still have a low atom economy.
Answer: it can make a lot of waste products
A student expects to make 12.5 g of a product but collects 10.0 g. Calculate the percentage yield.
Answer: 80%
Calculate the atom economy for making ammonia by N₂ + 3H₂ → 2NH₃. Mr of N₂ = 28, H₂ = 2 and NH₃ = 17.
Answer: 100%
Short questions with options. Choose one before you open the answer.
What is theoretical yield?
The maximum mass of product that could form, from the equation.
What is actual yield?
The mass of product you really collect.
What is the equation for percentage yield?
Actual yield ÷ theoretical yield × 100.
Can percentage yield be more than 100%?
No. A larger value means a mistake, or the product was impure or wet.
Give one reason why yield is less than 100%.
The reaction is reversible, product is lost, or side reactions happen.
What is a side reaction?
A different reaction that uses up reactants to make other products.
What is atom economy?
The share of the mass of the reactants that ends up in the desired product.
What is the equation for atom economy?
Mr of desired product ÷ sum of Mr of all reactants × 100.
Which type of reaction has 100% atom economy?
One with a single product, such as addition.
Why does a high atom economy help sustainability?
Less waste is made and fewer raw materials are used.
Can a reaction have a high yield but low atom economy?
Yes, if it makes a lot of waste products.
What do you use to find the theoretical yield?
The balanced equation and the moles of the limiting reactant.
What is the atom economy of CaCO₃ → CaO + CO₂ for CaO?
56%.
What does the ratio in the equation tell you?
The moles of product that form for each mole of reactant.
What is a waste product?
A product that is not the one you want.
What is a limiting reactant?
The reactant that runs out first.
How do you find moles from mass?
Moles = mass ÷ Mr.
How do you find mass from moles?
Mass = moles × Mr.
What is the percentage yield if the actual yield equals the theoretical yield?
100%.
What is a reversible reaction?
One that can go both ways, so it may not finish.
Name one way product can be lost during an experiment.
It is left on filter paper or stuck to the equipment.
Why multiply each Mr by the number in front of it in atom economy?
The equation shows how many of each formula react or form.
What is the atom economy of making ethanol from ethene and steam?
100%, because there is only one product.
What does the desired product mean in atom economy?
The product that you want to make.
Why can the percentage yield of a reaction never be more than 100%?
You cannot collect more product than the equation says is possible. A result over 100% means the product is impure or wet.
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