When 2.50 g of copper reacts with oxygen the copper oxide product has a mass of 2.81 What is the simplest formula of the copper oxide?

(2.5 grams =) 0.0393 moles of copper reacts with (0.31* grams =) 0.0194

moles of oxygen to produce something that has a ratio of 2.0 moles copper to 1 mole oxygen.

Copper has a +1 and a +2 oxidation state and Oxygen has (usually) a -2 oxidation state so we know it isn't CuO because that's a 1:1 ratio. Cu2O has a 2:1 ratio...what could it be?

*notice that the subtraction: 2.81g - 2.50 g = 0.31 g, reduced my significant digits to two.