The equilibrium constant, Kc, for the equilibrium

Question

N2O4(g)←→2NO2(g)
is 4.6×10−3 .

If the equilibrium mixture contains 5.0×10−2M NO2, what is the concentration of N2O4?

Answer 1

Kc = [NO2]^2 / [N2O4]

so [N2O4] = [NO2]^2 / Kc

Answer 2

........N2O4 ==> 2NO2
E.......x.........5E-2M

Kc = 4.6E-3 = (NO2)^2/(N2O4)
Substitute and solve for N2O4.