Chemistry • Year 11 • Module 2 • Lesson 14
Percentage Yield & Percentage Purity
Secure the two key formulas, identify when each correction applies, and practise the vocabulary and concepts before tackling harder calculations.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: percentage yield, theoretical yield, actual yield, percentage purity, mass of pure substance, assay, side reaction, limiting reagent. 8 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The maximum mass of product predicted by stoichiometry from the moles of limiting reagent available; calculated using Steps 2–4 of the four-step method. | |
| 1.2 | The mass of product actually collected in an experiment; always less than or equal to the theoretical value. | |
| 1.3 | (actual yield ÷ theoretical yield) × 100; measures how efficient the reaction was; always between 0% and 100%. | |
| 1.4 | (mass of pure substance ÷ mass of sample) × 100; expresses what fraction of a sample is the desired substance. | |
| 1.5 | m(sample) × (% purity ÷ 100); the mass that should be used as the starting value in stoichiometric Step 2 when an impure reactant is given. | |
| 1.6 | The determination of the purity of a substance; widely used in pharmaceutical and industrial quality control. | |
| 1.7 | An unwanted reaction that consumes reactant and produces a by-product, thereby reducing the yield of the desired product. | |
| 1.8 | The reactant that is completely consumed first and therefore determines the theoretical yield; whichever gives the fewest moles of product. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Percentage yield can exceed 100% if the reaction is highly efficient. T / F
2.2 Percentage purity is a property of the starting material and must be applied before stoichiometric calculations. T / F
2.3 To find the usable mass of a reactant from an impure sample, multiply the sample mass by (% purity ÷ 100). T / F
2.4 Percentage yield is calculated by dividing the theoretical yield by the actual yield and multiplying by 100. T / F
2.5 A reaction can have a yield of less than 100% due to product losses during filtration or purification steps. T / F
2.6 When an impure reactant is used in a stoichiometric calculation, the sample mass (not the pure mass) should be used in Step 2 to calculate moles. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
after · before · 100 · actual · impurities · purity · sample · theoretical
Percentage yield measures the efficiency of a chemical reaction. It is calculated ___________ stoichiometry by comparing the ___________ yield (experimentally measured) to the ___________ yield (calculated from the limiting reagent). The result is always between 0 and ___________%, because you cannot collect more product than stoichiometry predicts. Percentage ___________, by contrast, tells you the fraction of a ___________ that is the desired substance. It must be applied ___________ stoichiometry, because ___________ in the reactant reduce the effective number of moles available to react.
4. Function recall
Answer each question in 1–2 sentences using precise chemical terminology. 8 marks (2 each)
4.1 What does percentage yield measure, and at which stage of a stoichiometric calculation is it applied?
4.2 Why must a purity correction be applied before the moles calculation (Step 2) rather than after?
4.3 List two reasons, other than impure reactants, why percentage yield is less than 100%.
4.4 What does it mean if a calculated percentage yield comes out above 100%, and what is the most common cause?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is applied before”, “reduces”, “is calculated from”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: percentage purity · pure mass · stoichiometry · theoretical yield · actual yield · percentage yield.
Q1 — Term–definition match
1.1 theoretical yield • 1.2 actual yield • 1.3 percentage yield • 1.4 percentage purity • 1.5 mass of pure substance • 1.6 assay • 1.7 side reaction • 1.8 limiting reagent.
Q2 — True / false with correction
2.1 False. Percentage yield cannot exceed 100% because you cannot collect more product than stoichiometry allows. A value above 100% indicates an error in the calculation or the data (e.g. actual and theoretical swapped, or impurities inflating the actual mass).
2.2 True.
2.3 True.
2.4 False. Percentage yield = (actual yield ÷ theoretical yield) × 100 — the actual yield is in the numerator, not the denominator. Dividing theoretical by actual gives a number >1 (above 100%), which is impossible for a real yield.
2.5 True.
2.6 False. The pure mass (not the sample mass) must be used in Step 2. Using the sample mass overestimates the moles available because the impurities do not react.
Q3 — Cloze paragraph
In order: after / actual / theoretical / 100 / purity / sample / before / impurities.
Q4.1 — Percentage yield — what and when
Percentage yield measures how efficient a chemical reaction was by comparing the mass of product actually collected to the maximum mass predicted by stoichiometry. It is calculated after stoichiometry, using the result of the four-step calculation as the theoretical yield.
Q4.2 — Why purity before moles
Only the pure fraction of the sample can react. If the purity correction is not applied first, the moles calculation (Step 2) uses the full sample mass, overestimating the moles of reactant available. This inflates every subsequent step and gives a theoretical yield that is too high.
Q4.3 — Two reasons yield < 100%
Any two of: (1) Side reactions consume reactant and divert it away from the desired product. (2) Incomplete reaction — not all reactant molecules collide with sufficient energy to react. (3) Reversible reaction — equilibrium is reached before reactants are fully consumed. (4) Product loss during transfer, filtration, evaporation, or purification.
Q4.4 — Yield above 100%
A calculated yield above 100% indicates an error — in a real reaction, you cannot produce more product than stoichiometry allows. The most common cause is swapping actual and theoretical yield in the formula (dividing theoretical by actual instead of actual by theoretical).
Q5 — Sample concept map
Accept any six valid labelled arrows. Examples: percentage purity → is used to calculate → pure mass; pure mass → is the input for → stoichiometry; stoichiometry → gives → theoretical yield; actual yield → is compared to → theoretical yield; actual yield → divided by theoretical yield gives → percentage yield; percentage purity → is applied before → stoichiometry. Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).