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Chemistry Question on Enthalpy change

At 25C25^{\circ} C, the enthalpy of the following processes are given: H2(g)+O2(g)2OH(g)ΔH=78kJmol1H _2( g )+ O _2( g ) \rightarrow 2 OH ( g ) \Delta H ^{\circ} =78 \,kJ \,mol ^{-1} H2(g)+1/2O2(g)H2O(g)ΔH=242kJmol1H _2( g )+1 / 2 O _2( g ) \rightarrow H _2 O ( g ) \Delta H ^{\circ} =-242 \,kJ \,mol ^{-1} H2(g)2H(g)ΔH=436kJmol1H _2( g ) \rightarrow 2 H ( g ) \Delta H ^{\circ} =436\, kJ \,mol ^{-1} 1/2O2(g)O(g)ΔH=249kJmol11 / 2 O _2( g ) \rightarrow O ( g ) \Delta H ^{\circ} =249\, kJ\, mol ^{-1} What would be the value of XX for the following reaction?______ (Nearest integer) H2O(g)H(g)+OH(g)ΔH=XkJmol1H _2 O ( g ) \rightarrow H ( g )+ OH ( g ) \Delta H ^{\circ}= X\, kJ\, mol -1

Answer

correct answer is 499.