What percentage composition of Oxygen is H2O

33.333% continued. In water the ratio is 2 H for every 1 O therefor the answer is 1/3 aka 33.333% continued.

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.