Question

If 100 grams of copper (cu) completely react with 25 grams of oxygen to form only cuo with no reactant left over, how many grams of cuo will form from 140 grams of copper and excess oxygen?

Answer 1

To form CuO - 100 g of Copper reacts with 25 g of Oxygen
then amount of CuO formed - 125 g 
Since excess oxygen is provided, Copper is the limiting reactant. Then Copper is completely consumed during the reaction and amount of product formed depends on the amount of Copper used.
So if 100 g of Copper makes up 125 g of CuO
Then 140 g of Copper will form - 125/100 x 140 = 175 g of CuO