How much energy must be added to 2 mil h2o(g) to cause it to be decomposed into h2 and o2

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); = 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.