$\mathrm{S}+{\mathrm{O}}_2\stackrel{∆}{\to }{\mathrm{SO}}_2,$ the percentage of ${\mathrm{SO}}_3$ formed along with ${\mathrm{SO}}_2$ is

${\mathrm{SO}}_3$ cannot be obtained by direct combination of sulphur and oxygen;

${\mathrm{SO}}_2$ reacts with ${\mathrm{O}}_2$ at 723K in presence of catalyst to give good yield of ${\mathrm{SO}}_3$;

When sulphur and oxygen are burnt in absence of catalyst , ${\mathrm{SO}}_2$ is formed;

During this process small amount ${\mathrm{SO}}_2$ oxidizes to give 6-8% of ${\mathrm{SO}}_3$;