[katie_nanook]

OK so, I'm quite bad at physical chemistry and need some help for my exams coming up.
Can someone help me with this question? 

1 mole of nitrogen gas at 250 K is compressed isothermally and reversibly from an initial volume of 24 dm^3 to a final volume of 4 dm^3. Assuming the gas obeys the ideal gas equation calculate ΔH, the increase in enthalpy of the gas.

Answer has no units.

I know the answer is zero, but why?? 

[cseil]

For an isothermal transformation, ΔU is always 0 since U depends only on temperature.
ΔH = ΔU + ΔPV = ΔU + RnΔT.

ΔT is 0 so ΔH = 0.