In a closed container and at constant temperature $0.3$ mole of ${\mathrm{SO}}_2\&0.2$mole of ${\mathrm{O}}_2$gas at 750 torr are kept with a catalyst. If at equilibrium $0.2$mole of ${\mathrm{SO}}_3$ is formed the partial pressure of ${\mathrm{SO}}_2$ is ___ torr

It is given to find the partial pressure of ${\mathrm{SO}}_2$. In a closed container and at constant temperature $0.3$mole of ${\mathrm{SO}}_2\&0.2$ mole of ${\mathrm{O}}_2$ gas at 750 torr are kept with a catalyst. At equilibrium$0.2$ mole of ${\mathrm{SO}}_3$ is formed.

 ${\mathrm{SO}}_2+\frac{1}{2}{\mathrm{O}}_2\leftrightharpoons {\mathrm{SO}}_3$

Mole fraction of a component $=\frac{\text{ moles of that component }}{\text{ total number of moles }}$

Mole fraction of ${\mathrm{SO}}_2=\frac{0.1}{0.1+0.1+0.2}=\frac{0.1}{0.4}=0.25$

Partial pressure of ${\mathrm{SO}}_2=$ mole fraction of ${\mathrm{SO}}_2\times$ total pressure

Partial pressure of ${\mathrm{SO}}_2=0.25\times 750$ torr $=187$ torr

Hence the correct option is (B) 187.