a If the pressure of the system is increased, according to Le Chatelier's principle, the equilibrium will shift towards the side with fewer moles of gas to counteract the change. In this case, there are 3 moles of gas on the reactant side 2 moles of SO2 and 1 mole of O2 and 2 moles of gas on the product side 2 moles of SO3 . Therefore, when the pressure is increased, the equilibrium will shift towards the product side, and the concentration of SO3 will increase.b If the concentration of SO2 is increased, the equilibrium will shift towards the side that consumes the added substance to counteract the change, according to Le Chatelier's principle. In this case, increasing the concentration of SO2 will cause the equilibrium to shift towards the product side, and the concentration of SO3 will increase.c Adding a catalyst to the system will not affect the equilibrium concentrations of the reactants and products. A catalyst only speeds up the rate at which the reaction reaches equilibrium by providing an alternative pathway with a lower activation energy. It does not change the position of the equilibrium itself. Therefore, the concentration of SO3 will remain unchanged when a catalyst is added.