Finding value of equilibrium constant I tried to find the value of equilibrium constant several times.
I failed.. 

the value of equilibrium constant at 500K
has enthalpy 250kJ/mol and entropy 48J/mol*K

the answer is $ 2.45*10^{-24} $

which formula do I have to use?
$\Delta G=\Delta H - T \Delta S = 250-48 \times 10^{-3} \times 500 = 226 KJ/mol$
$K=exp(-\frac{\Delta G}{RT})=exp(-\frac{226 \times 10^3}{8.314 \times 500})=2.45 \times 10^{-24}$