At $900K,$ the reaction of NiO with $C{l}_2$ gas is

$Ni{O}_{(s)}+C{l}_{2(g)}=NiC{l}_{2(s)}+\frac{1}{2}{O}_{2(g)}$

and

$G^0=-15,490J$

Calculation the total pressure as conversion rate of $C{l}_{2(g)}$ is $90\%.$