For a dimerization reaction, $2\mathrm{A}\left(\mathrm{g}\right)\to {\mathrm{A}}_2\left(\mathrm{g}\right)$, at 298 K, 

$\mathrm{\Delta }U\stackrel{\ominus }{=}-20{\mathrm{kJmol}}^{-1},\mathrm{\Delta }S\stackrel{\ominus }{=}-30{\mathrm{JK}}^{-1}{\mathrm{mol}}^{-1}$, then the - $\mathrm{\Delta }{G}^{\mathrm{\Theta }}$ will be ________ J.