Isn't SO3 formed from SO2 ONLY in the presence of vanadium V oxide and the question says "BURNED" in excess oxygen ie combustion....Q20 i cant think of any reasonable explanation for this question other than the OH- being a nucleophile
as for Q12 ... they said 1 mole of Mg , Al and S so write down all 3 combustion equations and balance but keeping 1 mole of those 3 .. 1) Mg + 1/2 o2--> MgO 2) Al +3/4O2--> Al2O3 3) S+3/2O2-->SO3 notice the moles of O2 in those combustion eqns are increasing but not equally thus the only graph representing this is D