Percentage yield compares the mass of pure product actually obtained with its theoretical maximum. Divide actual yield by theoretical yield and multiply by 100. The theoretical mass comes from the balanced equation and assumes complete conversion of the limiting reactant.
Atom economy describes the reaction equation. Multiply the coefficient of each reactant by its relative mass and add these contributions. Then divide the coefficient-weighted mass of the desired product by this total and multiply by 100.
Atom economy and percentage yield answer different questions. For a reaction with only one product, atom economy is 100%. Some product may still be lost during transfer or separation, making the percentage yield smaller.
A wet or impure sample may appear to give an unusually high measured yield. Such a mass cannot be treated as the mass of pure desired product. It does not change the equation or its atom economy.
Both boards assess percentage yield and calculated atom economy at Foundation and Higher tier. AQA Higher adds calculating the theoretical mass from a reactant and the balanced equation. Edexcel and AQA reserve certain more demanding pathway-evaluation work for Higher tier; this exercise keeps that separate.
Worked example
CaCO₃ → CaO + CO₂. The equation contains 100 parts by relative mass of CaCO₃ and makes 56 parts of CaO plus 44 parts of CO₂. If CaO is the desired product, atom economy is 56 ÷ 100 × 100 = 56%.
If the maximum CaO mass is 28.00 g but only 21.00 g of pure, dry CaO is collected, percentage yield is 21.00 ÷ 28.00 × 100 = 75%. The 56% atom economy is unchanged.
Specification references
AQA 8462 4.3.3.1–4.3.3.2; Pearson Edexcel 1CH0 Issue 4 5.11C–5.15C. Select your course to see the tier of each question.
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Explain why a reaction can have 100% atom economy but a percentage yield below 100%.
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Define atom economy using the balanced equation.
Define percentage yield using actual and theoretical pure product mass.
Give one plausible reason for product recovery to be incomplete.
Example answer
Atom economy measures what fraction of the mass in the balanced equation ends up in the desired product. If there is only one product, atom economy can be 100%. Percentage yield compares the pure product obtained with the maximum calculated mass. Some product may be lost during filtration, so the percentage yield can still be below 100%.
An answer to improve
A 100% atom economy means you always get every gram of product in the flask.
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Keep the calculation from the equation separate from the mass of product collected.
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