Basic facts on Hydrogen
H2
When water is electrolyzed, i.e. cut up with electricity, it forms oxygen and hydrogen gas.
2H2O(l) ® O2(g) + 2H2(g)
For every volume of oxygen produced, two volumes of hydrogen are produced. Even though oxygen is a heavier gas, all gases occupy an equal volume, no mater what their weight is. Now hydrogen is a very light gas, just think of the Hindenburg...
Lindsay Leveen has just written an excellent book on Hydrogen usage that he is in the process of getting published.