1. Percentage Composition
The percentage by mass of each type of element in a compound.

2. Percentage % element = X 100
(# of atoms of that element in formula) (atomic weight of that element)
X 100
Molar mass of the compound

3. 1. Calculate the percentage of both hydrogen and oxygen in water.
(2)
1.0 g
= 2.0 g
Part O
(1)
16.0 g
= 16.0 g
Part
18.0 g
Whole
2.0 g
% H =
x 100 =
11.1%
18.0 g
16.0 g
% O =
x 100 =
88.9%
18.0 g

4. 2. Calculate the percentage of both carbon and oxygen in carbon dioxide.
(1)
12.0 g
= g
Part O
(2)
16.0 g
= 32.0 g
Part
44.0 g
Whole
12.0 g
% C =
x 100 =
27.3%
44.0 g
32.0 g
% O =
x 100 =
72.7%
44.0 g