`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

`H_(2)(g)+1/2O_(2)(g)to H_(2)O_((1)),Delta H-286.2KJ` …….(1)
`H_((aq))^(+) +OH_((aq))^(-) to H_(2)O_((1)), DeltaH^(@)=-57.3KJ` ……(2)
`1/2H_(2)(g) +aq to H_(aq)^(+) +e^(-),Delta H=0` …….(3)
(1)-(2)-(3), We get the value of standard heat of formation of `OH_((aq))^(-)`
`1/2H_(2)(g) +1/2O_(2)(g)+aq to OH_((aq))^(-)`,
`Delta H = -228.9KJ mol^(-)`