Dinitrogen tetraoxide decomposes to give nitrogen dioxide. Calculate the equilibrium concentration of NO2 at 100oC if the equilibrium constant is 0.200 and [N2O4] = 0.800M.

Question

N2O4(g) ¡ê 2 NO2(g)

Answer 1

You have some strange symbols which I assume was supposed to be an arrow. I've corrected it. Do you mean [N2O4] = 0.800M at equilibrium? or initially? I'll assume at equilibrium.
...........N2O4(g) ==> 2NO2(g)

Kc = (NO2)^2/(N2O4
0.200 = (NO2)^2/(0.800.
Solve for (NO2).