Answer
0.35
Work Step by Step
We find:
$\Delta G^{\circ}=-RT\ln K_{p}$
$\implies \ln K_{p}=-\frac{\Delta G^{\circ}}{RT}=-\frac{2.60\times10^{3}\,J/mol}{(8.314\,Jmol^{-1}K^{-1})(25+273)K}$
$=-1.0494$
$\implies K_{p}=e^{-1.0494}=0.35$