If Cs+O2g   →  CO2g,   ∆H=X  and  COg+1/2O2g   →  CO2g,   ∆H=Y,  then the heat of formation of CO is

Cs+O2g   →  CO2g,   ∆H=X        ....(1)COg+1/2O2g   →  CO2g,   ∆H=Y      ....(2) Subtracting equation (2) from equation (1), we getCs+12O2g   →  COg=0or  Cs+12O2g   →  COg,   ∆H=X-Y  ∆H=X-Y= ∆HfCO-0+0 ∆H4CO = X-Y