If bond dissociation energies of $xy,x_2$ and $y_2$ (all diatomic molecules) are in the ratio of 1 : 1 : 0.5 and $\Delta H_f$ for the formation of $xy$ is$-200KJmo{l}^{-1},$ the bond dissociation energy of $X_2$ will be:

$X_2+Y_2\to 2XY$

$\Delta H={\left(BE\right)}_{x-x}+{\left(BE\right)}_{y-y}-2{\left(BE\right)}_{x-y}$

$\text{ If B.E of }(x-y)=\mathrm{a}\text{, then B.E. of }(x-x)=\mathrm{a}$

and B.E of $(Y-y)=\frac{a}{2}$

$\Delta H(x-y)=200kJ$

$\therefore$ 400 ( for 2 moles of xy) $=a+\frac{a}{2}-2a$

$400=\frac{a}{2}or=+800kJ$

The bond dissociation energy of  $x_2=800kJ/mol$