Percentage Yield – IGCSE Chemistry Definition
IGCSE Chemistry definition of percentage yield: the actual yield as a percentage of the theoretical yield. Covers the formula, reasons for low yield, and calculations.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Percentage yield measures how efficient a reaction is by comparing what you actually obtain to what you theoretically could obtain. It is a Supplement calculation on the 0620 syllabus and appears regularly on Paper 4. Understanding why yields are below 100% is also tested as a theory question.
The 0620 definition
Percentage yield = (actual yield / theoretical yield) x 100
- Actual yield: the mass of product actually obtained from the experiment
- Theoretical yield: the maximum mass of product calculated from the balanced equation (assuming the reaction goes to completion with no losses)
Why yield is less than 100%
| Reason | Explanation |
|---|---|
| Incomplete reaction | Not all reactants are converted to products |
| Side reactions | Reactants form unwanted by-products |
| Loss during transfer | Product lost when transferring between containers |
| Loss during purification | Product lost during filtration, washing, or drying |
| Reversible reactions | The reaction reaches equilibrium and does not go to completion |
Worked example
CaCO₃ → CaO + CO₂
A student heats 10.0 g of calcium carbonate and obtains 4.48 g of calcium oxide. Calculate the percentage yield. (Ar: Ca = 40, C = 12, O = 16)
-
Theoretical yield: moles of CaCO₃ = 10.0 / 100 = 0.1 mol. From the equation, 1 mol CaCO₃ gives 1 mol CaO. So 0.1 mol CaO expected. Mass = 0.1 x 56 = 5.6 g.
-
Percentage yield = (4.48 / 5.6) x 100 = 80%
Worked exam question
In a reaction, 6.5 g of zinc reacts with excess hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. The theoretical yield of ZnCl₂ is 13.6 g. The actual yield is 10.88 g. (a) Calculate the percentage yield. (1) (b) Give one reason why the actual yield is less than the theoretical yield. (1)
Mark scheme
(a) Percentage yield = (10.88 / 13.6) x 100 = 80% [1]
(b) Accept any valid reason: some product was lost during transfer / filtering / evaporation [1] or the reaction was incomplete [1]
Common exam mistakes
- Putting the values upside down in the formula. Actual yield goes on top, theoretical yield on the bottom.
- Confusing percentage yield with percentage purity. Yield compares actual product to theoretical product. Purity compares the mass of desired substance to total mass of sample.
- Forgetting to calculate the theoretical yield first. You must use the balanced equation and mole calculations to find what you should have obtained before applying the percentage yield formula.
Studying this yourself? Tutoring arrangements are normally made by a parent or guardian. Message us for the details to share with them, or send them this page.