Molecular mass of water , `H_(2) O = 2 + 16 = 18` amu .
Percentage of hydrogen
`H_(2)O` has two atoms of hydrogen
So , mass of hydrogen in `H_(2)O = 2` amu .
Mass of oxygen = 16 amu
`therefore` percentage of hydrogen = `(2)/(18) xx 100 = 11.1`
and percentage of oxygen = `(16)/(18) xx 100 = 88.9` .