Pierrotechnique

Alright so I'm stuck with this problem that's asking me to calculate the change in internal energy of a reaction at 25 degrees C and that has a enthalpy change of -3256 kJ/mol. So I'm thinking using :delta:U = :delta:H - RT(nf-ni) but then which value of R would I use, and on top of that it would introduce a pressure unit into the whole thing which I don't want because I want a final answer in kJ/mol. So where do I start to analyse this problem?