According to the law of constant composition, a pure chemical compound always consists of the same elements combined together in a fixed proportion by

Based on the Law of constant composition, the atoms of elements are always combined together in their atomic weights ratio to form a pure chemical compound.

Ex: Carbon dioxide is prepared by two methods.

Method 1: By combustion of Carbon - $C+O_2\to C{O}_2\uparrow$

Method 2: By heating Calcium Carbonate - $CaC{O}_3\stackrel{\Delta }{\to }CaO+C{O}_2\uparrow$

Here the ratio of Carbon and Oxygen is 3 : 8

$C{O}_2\Rightarrow Carbon:Oxygen$

                    $12g$   :  $32g$

                       $3:8$