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This lesson covers percentage yield and atom economy as required by the AQA GCSE Combined Science Trilogy specification (8464). You will learn how to calculate each, understand why they matter, and know the differences between them.
The percentage yield compares the amount of product you actually get in an experiment with the amount you theoretically could get (from the calculation).
percentage yield=theoretical yieldactual yield×100%
In practice, you almost never get 100% yield. The reasons include:
| Reason | Explanation |
|---|---|
| Incomplete reaction | The reaction may not go to completion — some reactant remains unreacted |
| Side reactions | Reactants may form unwanted by-products instead of the desired product |
| Loss during transfer | Product is lost when transferring between containers, filtering, or washing |
| Loss during purification | Some product may be lost during evaporation, crystallisation or drying |
| Reversible reactions | If the reaction is reversible, it reaches equilibrium and does not go to completion |
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