You must use 500 J to decompose 2 mol of H₂O(g).
The equation for the decomposition of water is
2H₂O(g) ⟶ 2H₂(g) + O₂(g); [tex]\Delta_{dec}H^{o}[/tex] = 241.82 kJ·mol⁻¹
Energy = 2 mol × (241.820 kJ/1 mol) = 500 J
Note: The answer can have only one significant figure because that is all you gave for the moles of the gas.
