S ( g )+3/2 O_2( g ) → SO_3( g )+2 x kcal; SO_2( g )+1/2 O_2( g ) → SO_3( g )+y kcal The heat of formation of SO_2( g ) is given by:

$$\begin{aligned} & \mathrm{S}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{SO}_3(\mathrm{~g})+2 x \mathrm{kcal} \\ & \mathrm{SO}_2(\mathrm{~g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{SO}_3(\mathrm{~g})+y \mathrm{kcal} \end{aligned} $$ The heat of formation of $\displaystyle \mathrm{SO}_2(\mathrm{~g})$ is given by :