The so-called hydrogen economy is based on hydrogen produced from water using solar energy...

The so-called hydrogen economy is based on hydrogen produced from water using solar energy. The gas is then burned as a fuel:

A primary advantage of hydrogen as a fuel is that it is nonpolluting. A major disadvantage is that it is a gas and therefore is harder to store than liquids or solids. Calculate the number of moles of H2 required to produce an amount of energy equivalent to that produced by the combustion of a gallon of octane (C8H18). The density of octane is 2.66 kg/gal, and its standard enthalpy of formation is –249.9 kJ/mol.