Sample of carbon dioxide taken from any source  will always have carbon and oxygen in the ratio 1 : 2  

When magnesium burns in oxygen, the amount of magnesium taken for the reaction is equal to the amount of magnesium in magnesium oxide formed

At constant temperature and pressure, 200 mL of  hydrogen will combine with 100 mL of oxygen to produce 200 mL of water vapour. 

Carbon forms two oxides namely $C{O}_2$  and  $CO,$ where masses of oxygen which combine with fixed mass of carbon are in the simple ratio 2 : 1