At $T(k),$ the partial pressures of $SO_2,O_2$ and $SO_3$ are $0.662,0.100$ and $0.331$ atm. respectively for the reaction $2 S0_2+O_2 \to 2 SO_3$ at equilibrium . What is the partial pressure in atm. of $O_2$ when the equilibrium partial pressures of $SO_2$ and $SO_3$ are equal at the same temperature?

Question

$$\begin {array} {1 1} (a)\;0.4 & \quad (b)\;0.8 \\ (c)\;0.25 & \quad (d)\;2.5 \end {array}$$