Define the Law of Multiple Proportions.

The Law of Multiple Proportions states that if two elements can combine to form more than one compound,the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
For example,carbon and oxygen can combine to form $CO$ and $CO_2$.
In $CO$,$12 \ g$ of carbon combines with $16 \ g$ of oxygen.
In $CO_2$,$12 \ g$ of carbon combines with $32 \ g$ of oxygen.
The ratio of the masses of oxygen that combine with a fixed mass $(12 \ g)$ of carbon is $16:32$,which simplifies to $1:2$,a simple whole number ratio.