If the copper was not completely dry, excess water is likely to be present in THE RESIDUE. As we know, water has its own mass, it will add to the overall mass of copper, making the actual yield higher causing an increase in percentage yield.

 

What else can i say in order to enforce my idea above.

 

Thank You

What are you talking about? This sounds like a standard high school experiment where you take copper, make copper nitrate, and reduce it back to copper.

If UC is correct in his assumption, you've answered the question pretty well.

 

One suggestion I'd make is to write that it causes an apparent increase in the percentage yield. because of course it hasnt changed how much copper you've got at all.