# 0.64 grams of copper when heated in air form 0.80grams of copper oxide. What is the simplest formula of copper oxide?

The moles of Cu reacted in excess oxygen is: mass / atomic mass of Cu = 0.01 mol. The moles of copper oxide formed will be some fraction (half, third, quarter, etc.) of 0.01 mol, depending on how the formula balances. Let’s call the big stochiometric coefficient in front of copper oxide ‘n’. The moles of copper oxide is therefore 0.01 x n, and the formula mass of copper oxide must be: mass / moles = 0.80 g / 0.01n = 80/n grams per mole. Remember, 64 g/mol of this 80 g/mol comes from a single Cu atom. The remaining 16 g/mol constitutes one O atom in each mole. So the simplest formula must be CuO, though Cu2O2, Cu3O3, etc. would result from using n values of 1/2, or 1/3, etc., rather than 1. This is what the question meant by ‘simplest formula’, because the others, while maintaining the correct molar ratio between Cu and O (1:1), do not accurately reflect the actual mass of the system (i.e. are multiples of the actual masses since there is an integer multiple of moles of the elements). I hope that helps!