Since water is
H
2
O
(molar mass is
18.015 g/mol
), it implies we have a percent composition of
H
and one for
O
.
Given that chemical formula, we have two equivalents of
H
, for which we can use its molar mass,
M
H
=
1.0079 g/mol
. We also have one equivalent of
O
, for which we can use its molar mass,
M
O
=
15.999 g/mol
.
The percent composition (by mass) of
H
is written in terms of a unified ratio of the molar masses is
%
H
=
2
×
M
H
M
H
2
O
×
100
%
=
2
×
1.0079 g/mol
18.015 g/mol
×
100
%
≈
11.19
%
H
Similarly for
O
, we have
%
O
=
1
×
M
O
M
H
2
O
×
100
%
=
15.999 g/mol
18.015 g/mol
×
100
%
≈
88.81
%
O
Thus, the percent composition of water, by mass, is
11.19
%
hydrogen and
88.81
%
oxygen.
