The formation of  ${\text{SO}}_2{\text{\hspace{0.17em}and\hspace{0.17em}SO}}_3$ explain

Law of multiple proportions, states that “when two elements combine to form more than one compound, the mass of one element, which combines with a fixed mass of the other element, will always be ratios of whole number”.

Ex: In the formation of ${\text{SO}}_2{\text{\hspace{0.17em}and\hspace{0.17em}SO}}_3$, combines with oxygen masses of

32 and 48 respectively, so the ratio of mass of oxygen is ${\text{SO}}_2{\text{\hspace{0.17em}and\hspace{0.17em}SO}}_3$ is 2:3.