The element carbon, combines with oxygen to form two different chemical compounds, which are, carbon dioxide and carbon monoxide. In CO2, 12 parts of carbon mass combine with 32 parts of oxygen mass while in CO, 12 parts of carbon mass combine with 16 parts of oxygen mass. Therefore, the masses of oxygen mix with a definite mass of 12 parts of carbon in CO2 and CO are 32 and 16 respectively. These oxygen masses bear a simple ratio of 32:16 or 2:1 to each other and this is a classic example of the law of multiple proportions.