The text is mistranscribed. [The original says...][1]

>Q: *Is there anything that contains hydrogen that is not a fuel?*  
A: *Yes, there is water, **the weight of which is one - eighth hydrogen***

Crucially, there's no ***of*** before ***hydrogen*** (or possessive ***'s*** *after* it).

Asimov isn't saying that water weighs only an eighth as much as hydrogen - he's saying that one eighth of the weight of water comes from the hydrogen it contains - along with oxygen, one atom of which which is 16 times heavier than a hydrogen atom (but water is H**2**O, and 16/2=**8**).

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EDIT: It's fully covered in comments, but in fact Asimov should have said that for any given weight of water, **1/9th** comes from the hydrogen it contains (16 from one atom of oxygen plus 2 atoms of hydrogen gives **18**, so hydrogen's contribution to "total weight" = 2/18 = **1/9**). But this doesn't affect the syntax.

  [1]: https://www.google.com/search?tbm=bks&q=%22Yes-there%20is%20water%2C%20the%20weight%20of%20which%20is%20one-eighth%20of%20hydrogen%22