Why we couldn't determine the empirical formula of copper (II) oxide by heating copper powder in a crucible??

Answer 1

Good idea. *. Shame it doesn't work.

If you could be certain that heating copper powder in a crucible converted all of it to CuO, then as you suggest the percentage of the original weight to final weight would give you the percent copper in the oxide, from which you could easily work out the empirical formula.

However, you would probably get incomplete conversion, with possible formation of Cu2O and possibly some remaining unreactive copper metal. The oxide would form a film over this and stop more air getting through.