Question: what is the percent by mass of oxygen in carbon dioxide?
Answer: This means that 1 mole of carbon dioxide has a mass of 44.01 g . Oxygen has a molar mass of 16.0 g mol−1 , so 1 mole of oxygen atoms has a mass of 16.0 g . Therefore, carbon dioxide has a percent composition of 72.7% oxygen, i.e. for every 100 g of carbon dioxide you get 72.7 g of oxygen.