For the equilibrium $2SO_2 + O_2 \rightleftharpoons 2SO_3$ we start with 2 moles of $SO_2$ and 1 mole of $O_2$ at 3 atm. When equilibrium is attained, pressure changes to 2.5 atm. Hence, $K_p$ is:

Question

(a) 3 $atm^{-1}$

(b) 2.5 $atm^{-1}$

(c) 2 $atm^{-1}$

(d) 0.5 $atm^{-1}$