5.0 g of calcium combines with 2.0 g of oxygen to give 7.0g of calcium oxide.Thus, by the law of definite proportions, when 5.00 g of calcium is burnt in 20.00 g of oxygen, only 2.00 g of oxygen will be used to produce 7.00 gram of calcium oxide. The remaining 18.00 g of oxygen will remain as is.