Let mol of both of $Cl _{2}$ and $Cl$ is $x$
$P _{ Cl }=\frac{ x }{2 x } \times 1=\frac{1}{2}$
$P _{ Cl _{2}}=\frac{ x }{2 x } \times 1=\frac{1}{2}$
$K _{ p }=\frac{\left(\frac{1}{2}\right)^{2}}{\frac{1}{2}}=\frac{1}{2}=0.5 \Rightarrow 5 \times 10^{-1}$
${I_2}(g)$ $\rightleftharpoons$ $2I(g),\;\Delta H_r^o(298\,K) = + 150\;kJ$