Percentage Yield and Purity Exam Questions

• Percentage yield compares the actual amount of product obtained to the maximum theoretical amount [1]
• Percentage yield = (actual yield / theoretical yield) x 100 [1]

Examiner note: The theoretical yield is the maximum amount of product calculated from the balanced equation. The actual yield is the amount obtained experimentally. The yield can never exceed 100%.

(a) Moles of CaCO3 = 50.0/100 = 0.5 mol [1] Mole ratio 1:1, so moles of CaO = 0.5 mol Mr of CaO = 56; theoretical yield = 0.5 x 56 = 28.0 g [1]

(b) Percentage yield = (21.0/28.0) x 100 [1] = 75.0% [1]

Examiner note: Show the reacting mass calculation first to find the theoretical yield, then apply the percentage yield formula. A common error is dividing by the mass of reactant (50.0 g) instead of the theoretical yield of product (28.0 g).

Any two from:

• The reaction may be incomplete / not all reactant is converted to product [1]
• Side reactions may produce unwanted by-products [1]
• Product may be lost during transfer between containers / during filtration / during purification [1]
• The reaction may be reversible, reaching equilibrium before completion [1]

Examiner note: “Errors in weighing” or “human error” are not valid reasons — they are sources of inaccuracy, not reasons for genuine yield loss. Focus on chemical or practical losses.

(a) Mass of CaCO3 = (80.0/100) x 10.0 = 8.0 g [1]

(b) Percentage purity = (mass of pure substance / total mass of sample) x 100 [1] = (7.5 / 10.0) x 100 = 75.0% [1]

Examiner note: Percentage purity = (mass of desired substance / total mass of sample) x 100. Note the difference from percentage yield: purity considers what fraction of the sample is the desired substance, while yield considers how much product you obtained compared to the theoretical maximum.

(a) Mr of Fe2O3 = 160; moles = 800/160 = 5.0 mol (in tonnes-equivalent) [1] Mole ratio Fe2O3 : Fe = 1:2, so moles of Fe = 10.0; mass = 10.0 x 56 = 560 tonnes [1]

(b) Actual mass = (92.0/100) x 560 [1] = 515.2 tonnes [1]

Examiner note: You can work in tonnes instead of grams as long as the units are consistent. Theoretical yield first, then multiply by the percentage yield (as a decimal or fraction). 92% of 560 = 515.2.

• Mole ratio Zn : H2 = 1 : 1, so moles of Zn reacted = 0.060 mol [1]
• Mass of pure Zn = 0.060 x 65 = 3.9 g [1]
• Percentage purity = (3.9/5.0) x 100 = 78.0% [1]

Examiner note: Only the zinc reacts — the impurities do not react with HCl (or produce different products). Work backwards from the moles of product to find the moles and mass of pure zinc, then calculate purity.

• Minimum percentage yield = (10.0/12.5) x 100 = 80.0% [1]
• To improve yield: ensure the reaction goes to completion (e.g. use excess reactant / heat the solution) [1]
• Or: minimise losses during crystallisation by careful transfer and thorough evaporation [1]

Examiner note: Improving yield means increasing the amount of product collected, not the theoretical amount. Practical measures include: using excess of one reactant, heating to ensure complete reaction, careful filtration, and thorough drying of crystals.