Chemistry equilibrium?

Question

Consider the reaction
2NO(g) + O2(g) = 2NO2(g)

at 430C an equilibrium mixture consists of 0.020 mole of O2, 0.040 mole of NO, and 0.96 mole of NO2. Calculate Kp for this reaction given that the total pressure is 0.20 atm.

I used PV=nRT to find the volume.
Total moles: 1.02
(.2 atm) V = 1.02 moles x 0.0821 x 703K
V= 294.35
Finding the Molarity for each : O2 - 6.79e-5, NO - 1.36e-4, NO2 - 3.26e-3

I plug it into the equation : (3.26e-3)^2 / (6.79E-5)(1.36e-4)^2

The answer comes out wrong.
The answer is suppose to be 1.5e5

what did i do wrong?

Answer 1

I believe that, if you are calculating M, you must be dealing with Kc but the problem gives you Kp.
I think what you need to do is to solve for mole fraction of each gas.
mole fraction NO = moles NO/total moles.
mole fraction O2 = moles O2/total moles.
mole fraction NO2 = mole NO2/total moles.
Then mole fraction oxygen x 0.2 = PO2.
mole fraction NO x 0.2 = PNO
mole fraction NO2 x 0.2 = PNO2.
Then substitute pressures into Kp expression to compute Kp.