Copper Recovery Lab: Limiting Reactant or Wet Product?
Ms. R is piloting a microscale copper-recovery lab for a 10th-grade chemistry class. Each pair receives 2.00 g of copper(II) sulfate pentahydrate, CuSO4·5H2O, and a 0.50 g iron nail. Students dissolve the hydrate in 25 mL of water, add the nail, and after 20 minutes decant the liquid. They rinse the remaining solid with distilled water and dry it overnight. The next day, Group A reports 0.42 g of copper and Group B reports 0.61 g. During the post-lab, one student argues that adding more iron would have increased the copper yield because “iron is the reactant that drives the reaction.” Another student says the group difference is just “experimental error” and cannot be analyzed. Ms. R wants the class to use the data to distinguish limiting-reactant reasoning from procedural error before students write their conclusions.
1. Determine the limiting reactant and the theoretical yield of copper in grams, showing the mole reasoning. Then state whether Group A’s or Group B’s mass is physically possible for pure, dry copper and explain.
Read the worked answer
M(CuSO4·5H2O) = 63.55 + 32.07 + 4(16.00) + 5(18.02) ≈ 249.7 g/mol; n = 2.00 g / 249.7 g/mol = 8.01 × 10−3 mol. n(Fe) = 0.50 g / 55.85 g/mol = 8.95 × 10−3 mol. The balanced reaction is Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s), a 1:1 mole ratio. CuSO4 is limiting because 8.01 × 10−3 mol requires only 8.01 × 10−3 mol Fe, and 8.95 × 10−3 mol Fe is available. Theoretical Cu = 8.01 × 10−3 mol × 63.55 g/mol = 0.509 g. Group A’s 0.42 g is 82.5% of theoretical, which is possible with transfer losses. Group B’s 0.61 g is 120% of theoretical, which is not possible for pure, dry copper from the stated starting materials; the product must be wet, contaminated, or otherwise misrepresented.
2. Group B’s 0.61 g exceeds the theoretical yield. Identify two plausible procedural explanations and describe a specific check or change that would test each explanation.
Read the worked answer
One explanation is that the copper was not dry: residual water adds mass. A check is to dry to constant mass-reweigh after an additional 10–15 minutes of drying and confirm the mass no longer decreases. Another explanation is that solid iron or iron-containing debris remained with the copper. A check is to examine the solid with a magnet or to remove the nail before decanting and rinse the product thoroughly; if magnetic bits are present, the mass is not pure copper. A third possibility is oxidation of copper to CuO or basic copper salts during drying, which adds oxygen mass; drying under gentle conditions or in a desiccator and comparing the color and mass would test this. The key point is that a yield above 100% signals a product-purity problem, not a successful excess of product.
3. A student claims that adding more iron would increase the copper yield. Evaluate this claim using limiting-reactant logic and propose one design change that would make iron the limiting reactant, including the minimum iron mass needed.
Read the worked answer
The claim is incorrect under the stated conditions. The CuSO4 is limiting, so once all 8.01 × 10−3 mol Cu2+ has been reduced, additional iron has no Cu2+ left to react with; extra iron only remains as unreacted solid and can contaminate the product. To make iron limiting, the teacher would reduce the iron mass below the stoichiometric requirement: 8.01 × 10−3 mol Fe × 55.85 g/mol = 0.447 g. Using, for example, 0.30 g Fe would make Fe limiting (0.30/55.85 = 5.37 × 10−3 mol), and theoretical Cu would then be 5.37 × 10−3 mol × 63.55 g/mol = 0.341 g. Alternatively, the teacher could increase the CuSO4·5H2O mass while keeping 0.50 g Fe. Either change creates a genuine limiting-reactant question.