The net reaction is,
12N2(g)+12O2(g)+12Br2((g))⇌NOBr(g)
Kc(net)=[NOBr][N2]1/2[O2]1/2[Br2]1/2
Considering the given equations;
2NO(g)⇌N2(g)+O2(g); equilibrium constant=2.4×1030
Exchange the reactants and products of above equation and take reciprocal of the equilibrium constant
N2(g)+O2(g)⇌2NO(g); equilbrium constant=(12.4×1030)
Divide above equation with 2 and take square root of equilibrium constant expression.
12N2(g)+12O2(g)⇌2NO(g)
Equilibrium constant=(12.4×1030)1/2=0.6455×10−15
[NO][N2]1/2[O2]1/2=K′c=0.6455×10−15...(i)
NO(g)+12Br2(g)⇌NOBr(g)
[NOBr][NO][Br2]1/2=K′′c=1.4...(ii)
Multiplying equations ( i) and (ii)
[NO][N2]1/2[O2]1/2×[NOBr][NO][Br2]1/2=[NOBr][N2]1/2[O2]1/2[Br2]1/2
Kc(net)=K′c×K′′c=0.6455×10−15×1.4=9.037×10−16