Question

Why is an excess of zinc used in the mass percent copper lab?

A) To minimize accuracy
B) To ensure complete reaction
C) To lower the reaction rate
D) To limit zinc usage

Answer 1

The excess of zinc is used in the mass percent copper lab to ensure complete reaction and accurate measurement of the mass percent of copper in the lab sample.

Step-by-step explanation:

The excess of zinc is used in the mass percent copper lab to ensure complete reaction. When a zinc rod is inserted into a beaker containing an aqueous solution of copper(II) sulfate, a redox reaction occurs. The zinc electrode dissolves to give Zn²+ ions, while Cu²+ ions are simultaneously reduced to metallic copper. The excess of zinc ensures that all the copper ions are reduced and that the reaction goes to completion, resulting in accurate measurement of the mass percent of copper in the lab sample.