The given reaction is,
N2O4(g)⇌2NO2(g)
Kp=0.492atm
T=300K
KP=KC⋅(RT)Δng
Δng= number of gaseous molecules of products – number of gaseous molecules of reactants.
Δng=1
By putting the values in the above equation,
⇒Kc=RTKp=0.082×3000.492=2×10−2