The equilibrium 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g) is established in a rigid 2.0 L vessel at a certain temperature, giving a total pressure of 1.5 atm. Helium is then added until the total pressure reaches 3.0 atm; temperature and volume remain constant. Determine the direction, if any, in which the equilibrium will shift after the helium is introduced.