Questions 22-24 refer to the following information. A group of students was asked to recover Cu(s) f — Stoichiometry Chemistry Question

STEPS:

1. Identify the chemical reaction occurring in Step 3:
In Step 3, a strip of solid zinc ($\text{Zn}(s)$) is added to the acidic solution containing dissolved copper ions ($\text{Cu}^{2+}$). Because zinc is more active (more easily oxidized) than copper, a spontaneous single-replacement (redox) reaction occurs where zinc dissolves into the solution and copper ions reduce to form solid copper metal:
$$\text{Zn}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{Cu}(s)$$
2. Analyze the experimental discrepancy:
The problem states that the final mass of the recovered solid was slightly *more* than the predicted theoretical yield. In gravimetric analysis, a measured mass that is higher than the predicted value indicates that the final solid product contains a contaminant or solid impurity.
3. Determine the source of the contaminant:
To drive the reaction to completion, zinc is typically added in excess. If some of the solid zinc reactant does not fully dissolve or is not physically separated from the precipitated copper metal, it will remain in the beaker as a solid.
4. Relate the contaminant to the measured mass:
During the final steps, the solid mixture of copper and any unreacted zinc is filtered, dried, and weighed together. Because the balance cannot distinguish between copper and zinc, the measured mass will include the mass of the unreacted zinc contaminant. This results in a final measured mass that is higher than the predicted mass of pure copper. This identifies Option C as the correct explanation.

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WHY_OTHERS_WRONG:

• Option A is incorrect: The complete dissolution of the solid copper phosphate ($\text{Cu}_3(\text{PO}_4)_2$) in $\text{HCl}(aq)$ during Step 2 is necessary to free the copper ions into the solution so they can undergo the subsequent redox reaction. Complete dissolution is required to achieve the predicted theoretical yield; it would not cause the recovered mass to exceed the predicted yield.
• Option B is incorrect: If some of the copper metal adhered to the funnel during filtration, that solid would not be transferred to the balance to be weighed. This loss of product would decrease the final measured yield, making the mass of recovered $\text{Cu}(s)$ *less* than predicted rather than more.
• Option D is incorrect: Adding excess sodium phosphate ($\text{Na}_3\text{PO}_4$) in Step 1 ensures that the precipitation of copper ions is complete. Any excess phosphate ions remain dissolved in the aqueous supernatant liquid as spectator ions. Because these dissolved ions are washed through the filter paper and discarded, they do not contaminate the final solid copper product or increase its mass in Step 3.