Sulfur dioxide (SO₂) gas reacts with oxygen (O₂) to produce sulfur trioxide (SO₃): 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g). A 5.5 L reaction vessel is charged with a mixture of SO₂ and O₂ at 300 °C. When the equilibrium was established, the mixture had the three gases at these partial pressures: P_SO2 = 1.15 atm, P_O2 = 2.35 atm, and P_SO3 = 0.855 atm. Determine K_p.