The change in enthalpy when that much hydrogen gas reacts completely is
So, you know how much energy is released when 1 mole of hydrogen gas reacts with 0.5 moles of oxygen to produce 1 mole of water -
The idea is that when fewer moles of each reactant take part in the reaction, you'll release less energy; likewise, when you have more moles of the reactants, you'll release more energy.
You need to determine exactly how many moles of hydrogen gas will react. To do that, use its molar mass
Assuming that all of the hydrogen gas reacts, i.e. you have enough oxygen present, the amount of energy released will be