Percent Yield Above 100% in a Copper-Cycle Lab
Students perform a copper-cycle sequence starting with 2.00 g of copper wire. They dissolve the copper in nitric acid, precipitate copper(II) hydroxide with sodium hydroxide, heat it to form copper(II) oxide, dissolve the oxide in sulfuric acid, and reduce the copper ions back to metal with zinc. After vacuum filtration, each group dries its product in a 105 °C oven for 10 minutes and weighs it on a two-pan balance tared with a weigh boat. Group A reports 2.18 g; Group B reports 1.62 g; Group C reports 0.84 g. Group A's product is still warm when weighed, and its filtrate was not washed with distilled water. Group B used tap water for the final wash. Group C added its zinc in one large piece, and some gray solid remains mixed with the product. The teacher has 25 minutes of the next class for analysis and wants students to distinguish a real percent-yield problem from a measurement artifact.
1. Calculate the percent yield for Group A and explain why a value above 100% does not by itself prove that more copper was produced.
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Group A's percent yield is (2.18 g / 2.00 g) × 100% = 109%. A yield above 100% cannot mean that the reaction created copper atoms, because copper is conserved in the cycle. It means the weighed product is not pure dry copper or the mass reading is inflated. Likely causes include residual water or rinse solvent, a warm sample causing convection currents on the balance, unreacted zinc or copper oxide mixed with the product, or insufficient washing of soluble salts. Weighing warm material is a particularly plausible artifact here because the sample came directly from a 105 °C oven. The teacher should have the group dry the product to constant mass, cool it in a desiccator, and reweigh. If the mass remains above 2.00 g, the solid should be examined for gray zinc or black oxide. The result should be treated as an upper bound, not as evidence of a successful synthesis.
2. For Group C, propose the most likely source of low yield and describe one quick test that would distinguish it from a filtration loss.
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Group C's low mass and gray solid point to incomplete reduction: a single large piece of zinc has limited surface area, so some zinc may remain unreacted and some copper ions may not be reduced. The gray solid could be residual zinc, or it could be copper coated with zinc. A quick test is to treat a small, weighed sample of the product with dilute hydrochloric acid. If gas bubbles form, zinc is present, because zinc reacts with acid while copper does not. After the acid treatment, the sample can be filtered, washed, dried, and reweighed; a significant mass loss supports residual zinc. By contrast, a filtration loss would put copper in the filtrate rather than in the collected solid. The teacher could check the filtrate for blue copper color or add base to see whether copper hydroxide precipitates. Both tests are simple and distinguish contamination from loss.
3. How should the teacher use these three results in class? State a grading or instructional decision and justify it.
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The results should be used as an error-analysis exercise rather than as a single yield score. Group A's 109% should be flagged as a measurement or purity problem, not as the best synthesis. Group B's 81% yield may reflect transfer losses or the use of tap water, which can leave residue or introduce ions, but it does not by itself show a chemical failure. Group C's 42% yield likely reflects incomplete reduction and should be diagnosed with the acid test. The teacher can grade the lab report on whether students identify a plausible cause, propose a diagnostic test, and explain how the cause would affect the mass. A reasonable instructional decision is to require all groups to report an adjusted mass after drying to constant mass and to explain why the raw value is or is not trustworthy. Penalizing yield alone would reward the contaminated, warm sample in Group A and mislead students about what percent yield means.