$X_2$ $\to$ $2X$ ;   $\Delta H$ = $+a KJ/mol$   ………..$(ii)$

$Y_2$ $\to$ $2Y$ ; $\Delta H$= $+0.5a KJ/mol$   ………..$(iii)$

$\frac{1}{2}\,\, \times \,\,(ii)\,\, + \,\frac{1}{2}\, \times \,\,(iii)\,\, - \,\,(i)\,$ કરતાં $,\,\frac{1}{2}\,\,{{\text{X}}_{\text{2}}} + \,\,\frac{{\text{1}}}{{\text{2}}}\,\,{{\text{Y}}_{\text{2}}}\, \to \,\,{\text{XY}}\,$

$\Delta {\text{H}}\,\, = \,\,\,\left( { + \,\,\frac{{\text{a}}}{{\text{2}}}\,\, + \,\frac{{0.5}}{2}\,a\,\, - \,\,a} \right)$  ${\text{ - 200}}\,\, = \,\,\frac{{\text{a}}}{{\text{2}}}\, + \,\,\frac{{0.5}}{2}\,\,a\,\, - \,\,a\,\,\,\,\therefore \,\,\,a\,\, = \,\,800$