1. Percent Composition
It helps to know the % of the components in a shirt because they affect how warm it is, whether it will need to be ironed, and how it should be cleaned.
You will learn how the
% of the elements
in a compound are
important in chemistry.

2. % Composition
% by mass = mass of element x 100
total mass
g/mol
g/mol

3. % by mass = mass of element x 100
total mass
% O = g O x 100 =
13.60 g
39.7 % O
% Mg = g Mg x 100 =
13.60 g
60.3 % Mg

4. % by mass = mass of element x 100
total mass
% C = g C x 100 =
44.11 g
81.68 % C
% H = g H x 100 =
44.11 g
18.32 % H
3 (12.01) (1.01) = g /mol

5. 17.04 g /mol
80.06 g /mol
NH3: %N = g N x 100 =
17.04 g
82.22 % N
NH4NO3: %N = g N x 100 =
80.06 g
35.00 % N

6. % Composition Calculate the mass of oxygen in 175 g of N2O4.
Find the decimal composition of the element.
2(14.01) + 4(16.00) = g/mol
O = =
92.02
0.696
2) Multiply decimal composition by total mass of
the compound.
0.696 x 175g =
122 g of O

7. % Composition Calculate the mass of aluminum in 425 g of Al2(SO4)3.
2(26.98) + 3(32.07) + 12(16.00) = g/mol
Al = =
342.17
0.158
0.158 x 425 g =
67.2 g Al

8. % Composition
Calculate the mass of carbon in a compound containing 92.2% C with a mass of 125 g.
0.922 x 125 =
115 g C

9. Quick Quiz!
1. The percent composition of a compound is found by dividing the mass of each element by the ________.
moles of each element
percent of each element
total mass of the compound
total number of elements

10. Quick Quiz.
Calculate the percent by mass of carbon in cadaverine, C5H14N2, a compound present in rotting meat.
67.4% C
58.8% C
51.7% C
68.2% C