${\Delta }_f{G}^0$ at 400 K for substance ‘ S ’  in liquid state and gaseous state are $+200.7kcalmo{l}^{-1}$  and  $+203kcalmo{l}^{-1}$  respectively. Vapour pressure of liquid ‘ S ’ at 400K is approximately equal to  $(R=2cal{K}^{-1}mo{l}^{-1})$

$$\begin{gathered} S_{\left(l\right)}\to S_{\left(g\right)}{K}_p=P_{vap} \\ \Delta G_{\mathrm{Re}action}^0={\Delta }_G^{F^0}\left(Vapour\right)-\Delta F{G^0}_{Liq} \\ \Delta G_{reaction}^0=203-200.7=2.3Kcal/mole \\ =2300Cal/mol \\ \Delta G_{reaction}^0=-RT\ln K_p \\ 2300=-2.303\times 2\times 400\log K_p \\ \log K_p=-1.24\Rightarrow K_p={10}^{-124} \\ ={10}^{-2}\times {10}^{0.76}=6\times {10}^{-2}atm \\ \ln K_p=-2.875,{\log }_{10} \\ {K}_p=-1=-1.24 \\ {K}_p=anti\log (-1)=0.1atm \\ ={10}^{-2}\times {10}^{0.76}=6\times {10}^{-12} \end{gathered}$$