A reaction system in equilibrium according to reaction : 2SO2(g)+O2(g)⇌2SO3(g) in one litre vessel at a given temperature was found to be 0.12 mole each of SO2 and SO3 and 5 mole of O2. In another vessel of one litre contains 32g of SO2 at the same temperature. What mass of O2 must be added to this vessel in order that at equilibrium 20% of SO2 is oxidised to SO3: