Questions: Percent Yield and Reaction Efficiency

Question 1

A student runs a reaction and calculates 112% yield. What is the most likely explanation?

• A. The reaction was unusually efficient and produced more product than the stoichiometric limit
• B. The student used the wrong limiting reagent in the theoretical yield calculation
• C. The isolated product contains impurities — residual solvent, unreacted starting material, or byproducts — that added mass
• D. Conservation of mass was violated under the reaction conditions

Answer: (C)

A percent yield above 100% is not physically possible — the theoretical yield is an absolute ceiling set by stoichiometry and conservation of mass. A value over 100% signals an error: the 'product' mass includes something extra, most commonly residual solvent that wasn't fully dried, unreacted starting material that wasn't washed away, or byproducts. The student should re-dry and re-weigh. Option B could cause an incorrect theoretical yield calculation, but that would give an unexpectedly high or low percent, not specifically >100%.

Question 2

A five-step synthesis has each individual step running at 85% yield. What is the overall percent yield of the complete synthesis?

• A. 85% — yield is determined by the lowest single step
• B. 17% — losses add across five steps
• C. 44% — losses multiply across five steps (0.85⁵ ≈ 0.444)
• D. 75% — the average yield across steps

Answer: (C)

Yields compound multiplicatively, not additively. Each step retains only 85% of the material from the previous step: 0.85 × 0.85 × 0.85 × 0.85 × 0.85 = 0.85⁵ ≈ 0.444, or about 44%. This is why organic chemists obsess over yield optimization even for 'good' yields — a 5-step synthesis at 90% per step gives only 59% overall, and at 80% per step gives only 33%.

Question 3

A percent yield of 100% is achievable with sufficiently careful laboratory technique.

• T. True
• F. False

Answer: False

100% yield is virtually unattainable in practice. Mechanical losses are unavoidable: some product always clings to glassware walls, is lost in transfers, remains dissolved in wash solvents, or is destroyed by side reactions. Conservation of mass still holds — the 'missing' product is physically somewhere — but it cannot all be recovered. Experienced chemists consider yields above 90% excellent for most reaction types. A reported yield of exactly 100% should itself raise suspicion that the product is not fully pure.

Question 4

A reversible reaction that reaches equilibrium before all reactants are consumed will produce a percent yield less than 100% even if no product is physically lost.

• T. True
• F. False

Answer: True

Incomplete reaction is one of the main reasons real yields fall short. If a reaction is reversible, it reaches an equilibrium state where both reactants and products are present simultaneously. The theoretical yield assumes complete conversion of the limiting reagent, but equilibrium stops the reaction before that point. Le Chatelier's principle can be used to push the equilibrium toward products (e.g., removing product as it forms), but some shortfall is inherent in reversible systems.

Question 5

A student obtains 110% yield and declares the experiment a success. What does this result actually indicate, and what should the student do next?

Model answer: A 110% yield is impossible — conservation of mass prevents producing more product than the stoichiometric maximum. The result indicates the isolated 'product' contains impurities that added mass. The student should re-dry the product completely (to remove residual solvent), confirm purity by melting point, TLC, or spectroscopy, wash away unreacted starting material if present, and recalculate yield from the pure product mass.

The theoretical yield is a hard ceiling derived from stoichiometry. Anything above it means the measured mass includes something that is not the desired product. Common culprits are residual solvent (not dried long enough), starting material that co-crystallized with the product, or inorganic salt byproducts not washed away. Reporting >100% without investigating is an error in laboratory practice.