Question

A mixture containing the same number of moles of SO2 and O2 is placed in a 2L container at 900K. The initial pressure is 1.90Atm. A reaction occurs leading to the formation of SO3: 2SO3 + O2 <--> 2SO3 After equilibrium is established the TOTAL pressure drops to 1.60 Atm. What is the equilibrium constant, Kp, at 900K?

Answer 1

Equal moles means equal pressures; therefore, initial pressure SO2 = 0.95 atm and initial pressure O2 = 0.95 atm. (You can go through PV = nRT to confirm this if you wish.).
2SO2 + O2 ==> 2SO3

initial:
SO2 = 0.95
O2 = 0.95
SO3 = 0

change:
SO2 = -2x
O2 = -x
SO3 = +2x

equilibrium:
SO2 = 0.95-2x
O2 = 0.95-x
SO3 = 2x
total pressure = 1.6 = 0.95-2x+0.95-x+2x and solve for x. Substitute these into the above ICE chart to find individual concns, substitute into Ka expression and solve for Kp.