Consider again reaction 4.1, which involves only hydrogen and oxygen atoms. When one mole of hydrogen molecules (two grams) combines with half a mole of oxygen molecules (16 grams) to form one mole of water molecules (18 grams), the energy given o? turns out to be 242,000 joules, assuming that the water comes out as a gas rather than as a liquid. (If the water comes out as a liquid, then the total energy released is greater: 286,000 joules.) Since there are 500 moles of hydrogen gas in a kilogram, this means that burning a kilogram of hydrogen gas releases 500

times as much energy, or 121 million joules (again, assuming that the water comes out as a gas, as is usually the case in a combustion process).

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