Pure PCl₅ gas is placed in a 2.00-L flask. After heating to 250°C the pressure of PCl₅ is initially 2.000 atm. However, the gas slowly but only partially decomposes to gaseous PCl₃ and Cl₂. When equilibrium is reached, the partial pressure of Cl₂ is 0.814 atm. Calculate K_p for the decomposition.