Percentage yield and percentage purity are two calculations that look similar but answer different questions. Yield asks, “how much product did I actually make compared with the maximum possible?” Purity asks, “how much of my sample is really the substance I want?” Both build on stoichiometry from The mole concept, chemical formula and equation, and both are common in Paper 2. This guide keeps them clearly apart and works an example of each.
Theoretical yield versus actual yield
The theoretical yield is the maximum mass of product you could get if the reaction went perfectly and nothing was lost. You calculate it from the balanced equation using the mole ratio. The actual yield is what you really collected in the lab, which is almost always less.
Real yields fall short for honest, examinable reasons: the reaction may be incomplete or reversible, product is lost when filtering or transferring, some reactant takes part in side reactions, or crystals stay stuck to the apparatus. Knowing these reasons is itself worth marks.
The percentage yield formula
percentage yield = actual yield ÷ theoretical yield × 100
Both masses must be in the same unit, and both must refer to the same product. A yield above the theoretical maximum is impossible, so any answer greater than 100 per cent signals a mistake, usually a wet or impure product, or an arithmetic slip.
Worked example: percentage yield
Calcium carbonate decomposes on strong heating: CaCO₃ → CaO + CO₂. A student heats 50.0 g of calcium carbonate and collects 22.4 g of calcium oxide. Find the percentage yield. (Relative atomic mass: Ca = 40, C = 12, O = 16.)
- Molar mass of CaCO₃ = 40 + 12 + (3 × 16) = 100 g mol⁻¹.
- Moles of CaCO₃ = 50.0 ÷ 100 = 0.500 mol.
- Mole ratio CaCO₃ : CaO is 1 : 1, so theoretical moles of CaO = 0.500 mol.
- Molar mass of CaO = 40 + 16 = 56 g mol⁻¹, so theoretical yield = 0.500 × 56 = 28.0 g.
- Percentage yield = 22.4 ÷ 28.0 × 100 = 80%.
The structure never changes: find the theoretical yield from the equation first, then divide the actual yield by it. You can drill this pattern with the percentage yield calculation page.
The percentage purity formula
Purity is about a sample, not a reaction. An impure sample contains the substance you want mixed with other material.
percentage purity = mass of pure substance ÷ mass of impure sample × 100
Worked example: percentage purity
A 5.00 g sample of impure sodium chloride is analysed and found to contain 4.25 g of pure sodium chloride. Find the percentage purity.
Percentage purity = 4.25 ÷ 5.00 × 100 = 85%.
So the sample is 85% sodium chloride and 15% impurities by mass. Often the mass of pure substance is not given directly, you find it from a reaction. For example, if an impure marble sample reacts with excess acid and you use the volume of carbon dioxide to work out the mass of calcium carbonate that reacted, that mass is your “mass of pure substance”. Working out that intermediate mass is exactly the skill on the mass of product or reactant page.
Keeping yield and purity apart
Students lose marks by mixing these up, so hold on to the distinction:
- Yield compares what you made against the theoretical maximum of a reaction. You need a balanced equation and a mole ratio.
- Purity compares the pure part of a sample against the whole sample. You need the mass of the pure substance within it.
A quick check: if the question mentions a reaction and “how much product,” think yield. If it mentions a sample described as “impure,” think purity.
Common mistakes to avoid
- Not balancing the equation before reading the mole ratio, which throws off the theoretical yield.
- Using the wrong molar mass, often by miscounting oxygen atoms in a formula like CaCO₃.
- Comparing different substances, the actual and theoretical masses must be for the same product.
- Forgetting the × 100 and reporting a decimal fraction instead of a percentage.
- Reporting a yield over 100 per cent and not stopping to find the error.
Percentage yield and purity reward neat, staged working: equation, moles, mass, then the percentage. Write each line down and the marks come one at a time, even if the final number is slightly off. If setting up the theoretical yield is where you stall, that is a common one for a teacher to fix quickly. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial, drilling these calculations against real past-paper questions until the method is second nature.
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