Answer:
A feasible error could have been the removal of the sample before all water evaporated.
Explanation:
In order to determine the percentage of water in an hydrate, an experiment that could be performed is the heating of the sample until the mass does not change. If the student heated the sample an insufficient amount of time, water will be present in the sample, thus reducing the percentage reported.
This is a single replacement reaction. So the final products are a metal and a salt with replaced cation. In this case, the final products are copper (metal) and iron sulfate (salt).