`H_(2(g)) + 1/2O_(2(g)) rarr H_2O_((l)) , DeltaH = -286.2KJ`
`H_2O_((l)) rarr H_((aq))^(+) + OH_((aq))^(-) , DeltaH = +57.3 KJ`
Enthalpy of ionization `OH^(-)` in aqueons solution is 

Explanation

On adding given equations, 
`H_(2(g)) + 1//lO_(2(g)) to H_((aq))^(+) + OH_((aq))^(-), DeltaH^(@) = -228.51 KJ` But for `H_((aq))^(+) , DeltaH^(@) = 0`
Therefore heat of ionisation of `OH^(-)` is `- 228.51 KJ.`