The 118% Copper Yield
In Chemistry II, students prepare 250.0 mL of 0.100 M copper(II) sulfate solution from CuSO4·5H2O (molar mass 249.68 g/mol) and then react 25.00 mL aliquots with excess iron nails to recover copper metal. The pre-lab calculation gives 6.24 g of the hydrate. One group weighs 6.30 g, dissolves it, and dilutes to the mark. After reacting a 25.00 mL aliquot, the group filters, rinses once with distilled water, dries briefly, and obtains 0.187 g of copper. The accepted theoretical mass for the aliquot is 0.159 g. The group reports a 118% yield and writes, 'We got extra copper because the reaction kept going.' A second group obtains 0.104 g and says its iron nail was 'probably not pure.' The teacher must decide whether to permit a redo, how to respond to the 118% claim, and what to ask each group before assigning a grade.
1. Calculate the theoretical copper mass for a 25.00 mL aliquot and evaluate the 118% result. What does a yield above 100% imply about the measurement, not the reaction?
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Moles of CuSO4 in 25.00 mL of 0.100 M solution: 0.100 mol/L × 0.02500 L = 0.00250 mol. Because the reaction is 1:1, theoretical Cu mass = 0.00250 mol × 63.55 g/mol = 0.1589 g, about 0.159 g. The reported 0.187 g gives 0.187/0.1589 = 1.177, or about 118%. A yield above 100% cannot mean the reaction created extra copper; mass conservation forbids that. It means the collected solid is not pure dry copper, the solution concentration was higher than assumed, or the balance/filter tare is wrong. The 6.30 g hydrate is only about 1% above the target, so it cannot explain an 18% excess. Likely contributors are residual water, filter paper mass, unwashed soluble salts, small iron fragments, or copper oxidation. The correct response is to question the measurement, not to praise a 'better than expected' reaction.
2. List the most likely procedural causes of the high and low results and explain what diagnostic questions or observations would distinguish among them.
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For the 118% group, high mass can come from wet copper, filter paper not tared, inadequate rinsing so sulfate or iron salts remain, small iron pieces passing into the solid, or copper oxide formation during drying. Ask: Was the filter pre-weighed? Did you dry to constant mass? Did you wash with several small portions of distilled water? Was the filtrate still blue? Did you remove every nail fragment before drying? For the 65% group, low mass suggests lost copper during decanting or filtration, incomplete reaction because iron was not truly in excess, copper left on the nail, or solution not mixed. Ask: Was any blue color left? Did you scrape the beaker? Did you transfer all solid? Was the nail mass actually in excess? The teacher should require observations, not accept 'impure nail' as an explanation. Comparing filtrate color, filter mass, and drying records distinguishes the causes.
3. Propose a revised procedure that would reduce this error and explain how you would assess students' understanding of limiting reactants without simply grading the final number.
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Revise by using a pre-weighed, labeled filter or a weighed evaporating dish; wash the copper with three small portions of distilled water; dry to constant mass and record each mass; remove all iron before drying; and confirm no blue color remains in the filtrate. If possible, standardize the copper solution rather than relying on the hydrate, or account for hydrate purity. For assessment, do not grade only the yield. Ask students to calculate moles of Cu2+ and Fe, identify the limiting reactant for a given nail mass, predict the maximum Cu mass, and explain why 118% is impossible under conservation of mass. Use a particle diagram or a short written prompt: 'A student obtains 115% yield. Give two measurement explanations and one reason the reaction itself cannot explain it.' This reveals whether students understand stoichiometry and gravimetric technique rather than whether they happened to get a clean precipitate.