For the reaction,SO2(g) + 12O2(g) ⇌ SO3(g),If KP=KC RTx where the symbols have usual meanings then, the value of x is (assuming ideality).

By using this formula, equilibrium constant,KP=KCRT∆n Where ∆n = (no. of moles of products) - (no. of moles of reactants)For the given reaction,SO2(g) + 12O2(g) ⇌ SO3(g),∆ng = 1-12+1=1-32=-12