Partial Pressure Equilibrium

[Teleologist]

At a particular T and total pressure of 1.5 atm, consider the dimerization of nitrogen dioxide after starting with a mix of 1.0 moles of NO2 and 3.5 moles of N2O4. If the equilibrium pressure of nitrogen tetraoxide is 1.0 atm, the EQ amount of NO2 is closest to ..?

Ptotal = 1.5 atm = P(NO2) + P(N2O4)

P(NO2) = 0.50 atm.
P(N2O4)= 1.0 atm

K = P(N2O4)/(P(NO2))^2 = 1/0.50^2 = 4

So now we have the equilibrium constant.

Q = [N2O4]/[NO2]^2 = 4 = 3.5 moles / (1.0 moles)^2 = 3.5 < 4 = K.

Q < K.

Product forming reaction favored.

4 = 3.5+x/(1-2x)^2.

Unfortunately, this doesn't give me the right answer. What's up?

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[Blueprint MCAT Tutor]

What are the answer choices and where's the question from?

 

[Blueprint MCAT Tutor]

Okay so this question was bugging me. After chewing on it a bit, I'm wondering if maybe the Ptot is supposed to be initial rather than final?

Anyway, scroll down in this thread to see what I did:

http://forums.studentdoctor.net/threads/specific-science-questions.1053558/