Asample of 254.2 grams of copper completely reacted with oxygen to form 286.2 grams of a copper oxide product. how many grams of oxygen must have reacted?

The balanced reaction between copper and oxygen to form the copper oxide can be written as follows:
 
                        2Cu + O₂ --> 2CuO

This signifies that every two moles of Cu will react with 1 mole of O₂. 

We are given with 254.2 g of Cu, solving for the amount of O₂ needed:
       (254.2 g of Cu)(1 mol/63.55 g of Cu)(1 mol O₂/2 mol Cu)(32 g O₂/1 mol O₂) = 64 g of O₂

We are also given that 286.2 g of copper oxide is formed, solving for the amount of oxygen needed for the reaction to form the product:
     (286.2 g of CuO)(1 mol CuO/79.545 g)(1 mol O2/2 mol CuO)(32 g O2/1 mol O2)
              = 57.57 g of O2

From the two calculated values, our answer would be the lesser one. This signifies that not all copper reacted to oxygen forming the copper oxide. 

ANSWER: 57.57 g of O₂