${N_2}{H_4}(l)\,+\,2{H_2}{O_2}(l)\,\, \to \,{N_2}(g)\,\, + \,4{H_2}O(l);\,\,{\Delta _r}H_1^o\, = \, - 818\,\,kJ/mol$
${N_2}{H_4}(l)\,\, + {O_2}(g)\,\, \to \,{N_2}(g)\,\, + \,2{H_2}O(l);\,\,{\Delta _r}H_2^o\, = \, - 622\,\,kJ/mol$
${H_2}(g)\,\, + 1/2{O_2}(g)\,\, \to \,{H_2}O(l);\,\,{\Delta _r}H_3^o\, = \, - 285\,\,kJ/mol$
......$kJ/mol$
$N _2( g )+4 H _2 O (l ) \rightarrow N _2 H _4( l )+2 H _2 O _2(l )$
$\triangle_{ r } H _1^{\circ}=818 \,kJ / mol \ldots(1)$
$N _2 H _4( l )+ O _2( g ) \rightarrow N _2( g )+2 H _2 O ( l ) ; \Delta_{ r } H _2^{\circ}=-622 kJ / mol \ldots\, (2)$
$\left[ H _2( g )+\frac{1}{2} O _2( g ) \rightarrow H _2 O ( l ) ; \Delta_{ f } H _3^{\circ}=-285 kJ / mol \right] \times 2....(3)$
For $H _2( g )+ O _2( g ) \rightarrow H _2 O _2( I )$
$\Delta_{ r } H ^{\circ}\left( H _2 O _2, l \right)=2 \times \Delta_{ r } H _3^{\circ}+\Delta_{ r } H _2^{\circ}-\Delta_{ r } H _1^{\circ}$
$=(2 \times-285)-622+818$
$=-374\, kJ$
Now, heat of formation of $1\, mole =\frac{-374}{2}=-187\, kJ / mol$
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