The rate of formation of ${\mathrm{SO}}_3$ in the reaction $2{\mathrm{SO}}_2+{\mathrm{O}}_2\to 2{\mathrm{SO}}_3$ is $100 \mathrm{g}{ \min }^{-1}$. Hence, rate of disappearance of ${\mathrm{O}}_2$ is

$2{\mathrm{SO}}_2+{\mathrm{O}}_2\to 2{\mathrm{SO}}_3$

Rate of formation of  SO₃ =100$\frac{ \mathrm{gm} }{\min }$ = $\frac{100}{80}\frac{\mathrm{mole}}{\min }$

Rate of disappearance of O₂ = ?
Rate of disappearance of   O₂ =

$-\frac{1}{1}\frac{\mathrm{d}\left({\mathrm{O}}_2\right)}{\mathrm{dt}}=+\frac{1}{2}\frac{\mathrm{d}\left({\mathrm{SO}}_3\right)}{\mathrm{dt}} = \frac{1}{2}\times \frac{100}{80}\mathrm{mole}/\min$

Rate of disappearance of   O₂ =

$= \frac{1}{2}\times \frac{100}{80}\times 32 \mathrm{gm}/\min$ 

= 20 gm /min