Question

Okay, I feel like this question should be really easy because it's a multiple choice question but how do you figure it out?? I'm completely drawing a blank.
Consider the following equilibrium
N2O4(g) <-> 2NO2(g)
Kc = 4.8E-3
Which set of concentrations represents equilibrium conditions?
a) [N2O4(g)] = 4.8E-1 and [NO2(g)] = 1.0E-4
And it goes to e) in the same format...
How do you find the answer?

I thought it might have something to do with
Kc = [NO2]^2 / [N2O4]
But I have two unknowns...

Answer 1

I don't think you have two unknowns; in fact, you don't have any unknowns.
Kc = ([NO2]^2/[N2O4]
Substitute the value given for NO2 and the value given for N2O4, perform the algebra (squaring the numerator and dividing by the denominator) and see which gives you 4.8E-3.