Ideally should the mass of copper at the start and finish of the exepriment be the same?

what law of chemistry applies to this situation?

As a practical matter, why would the two masses of copper metal be different?

Ideally the two masses of copper should be the same, this is explained by the Lavoisier's law or the law of conservation of mass. Lavoisier states that mass is neither created nor destroyed by chemichal reactions in an isolated enviroment.

But, as a practical matter, the masses should be a little different beacuse always a fraction of mass becomes energy.