It the bond dissociation energies of $\mathrm{XY}, {\mathrm{X}}_2$ and ${\mathrm{Y}}_2$ (all are gaseous diatomic molecules) are in the ratio of $1:1:0.5$ and ${\mathrm{\Delta H}}_{\mathrm{f}}$ for the formation of $\mathrm{XY}\text{ is }-200\mathrm{kJmol}$. The bond dissociation energy of ${\mathrm{X}}_2$ will be.

Since the bond dissociation energies of $\mathrm{XY},{\mathrm{X}}_2$ and ${\mathrm{Y}}_2$ are is the ratio $1:1:0.5$.

 so, let the bond dissociation  energies of $\mathrm{XY}, {\mathrm{X}}_2$ and ${\mathrm{Y}}_2$ are $\mathrm{x}, \mathrm{x}$and $\frac{\mathrm{x}}{2}$ respectively.

$\therefore \frac{1}{2}{\mathrm{X}}_2+\frac{1}{2}{\mathrm{Y}}_2⟶\mathrm{XY};{\mathrm{\Delta H}}_{\mathrm{f}}=-200 \mathrm{kJ} {\mathrm{mol}}^{-}$

$\therefore$ $\mathrm{\Delta H}\left(\text{ reaction }\right)=$ (Sum of bond dissociation energies of reactants) - (sum of bond dissociation energies of products)

$\begin{array}{l}\therefore \mathrm{\Delta H}\left(\text{ reaction }\right)=\frac{1}{2}∆{\mathrm{H}}_{{\mathrm{X}}_2}+\frac{1}{2}∆{\mathrm{H}}_{{\mathrm{Y}}_2}-∆{\mathrm{H}}_{\mathrm{XY}}\\ -200{\mathrm{kJmol}}^{-}=\frac{\mathrm{x}}{2}+\frac{0.5\mathrm{x}}{2}-\mathrm{x};\\ 2\times -200{\mathrm{kJmol}}^{-}=\mathrm{x}+0.5\mathrm{x}-2\mathrm{x};\\ -0.5\mathrm{x}=-400{\mathrm{kJmol}}^{-}\\ \therefore \mathrm{x}=\frac{400}{0.5}=800{\mathrm{kJmol}}^{-}\end{array}$