When trying to solve for the empirical formula of copper oxide, I got a ratio of 5 Cu for every 4 O, but I wasn't sure if it was actually a compound or not.
Copper forms two oxides Copper(I) oxide (cuprous oxide, Cu2O), a red powder; and Copper(II) oxide (cupric oxide, CuO), a black powder. This is because copper can donate either one or two electrons. What you probably got is a mixture of the two with 25% cuprous oxide and 75% cupric oxide.
