Percent by mass is an intensive property: it does not depend on the amount of material present.
Thus, we can calculate the percent by mass of one molecule of water.
#"Mass of 1 molecule" = 2 color(red)(cancel(color(black)("atoms H"))) × "1.008 u"/(1 color(red)(cancel(color(black)("atom H")))) + 1 color(red)(cancel(color(black)("atom O"))) × "16.00 u"/(1 color(red)(cancel(color(black)("atom O")))) = "2.016 u+ 16.00 u" = "18.016 u"#
#"% H" = "mass of H"/"total mass" × 100 % = (2.016 color(red)(cancel(color(black)("u"))))/(18.016 color(red)(cancel(color(black)("u")))) × 100 % = 11.19 %#
#"% O" = "mass of O"/"total mass" × 100 % = (16.00 color(red)(cancel(color(black)("u"))))/(18.016 color(red)(cancel(color(black)("u")))) × 100 % = 88.81 %#
