[Catherine S.P]

Im trying to work out the internal energy of the reaction:

H2 + 1/2 O2 --> H20

I have the delta H as =241kJmol-1, and I know that delta H=delta U + p deltaV, and you can sub in the ideal gas equation to give the equation delta H=delta U + deltaNRT.
The temperature is given as 298K.

Do you just assume that pressure is 100000Pa and then rearrange the equation and just plug the numbers in!?

[Schrödinger]

Yes. I guess that would be correct, but I don't think you will need any information about the pressure because you're not going to be using it anyway. All you have to do now is plug in the values of , R and T.

[Enthalpy]

So would I say as well.

And just to add my own word:

Enthalpy and internal energy of the reaction are defined at some temperature and pressure, and with some state of the reactants and products. For instance, the heat of formation of molecules refers often to the elements taken at 298K in their normal state, like gas molecules for oxygen, and graphite for carbon. The reference could have been atomized elements for instance, but this is less convenient to measure, lets manipulate unnecessary big quantities, and accumulate bigger uncertainties.

Various conventions are possible. Heat contents of fuels has two definitions: the higher, where produced water is condensed, and the lower, where it is vapour. Enthalpy of formation sometimes refers to gases at zero Kelvin, though this state doesn't exist.

For your computation of internal energy, since the variation of enthalpy is necessarily defined under some conditions (probably liquid water at 298K), you may take the pressure and volumes of the reactants and products under these same conditions to get the variation of internal energy, and then correct the result if needed under different conditions.