a student used a piece of copper with a mass of 2.85g and obtained 2.95g of copper at the end of the experiment, an apparent recovery of 104$. How could the student account for this result, since the best to expect is 100%.
A very general question which requires a very general answer.
The student made an error in weighing or some other part of the experiment. Another possibility is that the "chemistry" of the experiment did not go as expected.