65.5g of copper reacts with __g of oxygen to produce 81g copper (I) oxide

grams O₂ (g) ≅ 15.9 g O₂ (g) (3 sig. figs.)

Explanation:

2Cu + O₂ = > 2CuO

Convert given to moles, solve by equation rxn ratio then convert to grams.

Moles CuO formed = 81.0 g / 81.5 g·mol⁻¹ = 0.994 mol CuO

0.994 mol CuO requires 0.994 mol Cu = 0.994 mol Cu x 64 g·mol⁻¹ = 63.6 g Cu = > from this, the amount of O₂ needed = 1/2 (0.994 mol) O₂ (g) = 0.497 mol O₂ (g) = (0.497 mol O₂ (g)) (32 g·mol⁻¹) = 15.904 g O₂ (g) ≅ 15.9 g O₂ (g) (3 sig. figs.)