Question: how to find empirical formula of this oxide?
Answer: Determine the masses. Mass of Cu = 2.50 g. Mass of O = (3.13 – 2.50) g = 0.63 g.
Determine the moles. Moles of Cu = 2.50 g Cu × 1mol Cu63.55g Cu = 0.3934 mol Cu. ... 
Determine the Molar Ratios. Moles of CuMoles of O=0.3934mol0.394mol=11.00≈11.
Write the Empirical Formula. The empirical formula of the oxide is CuO.