1. Unit 8 Chemical 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
Cr:
52.00 g
Cr: g

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
C3H8: 3 (12.01) (1.01) = g/mol
% C = g C x 100 =
44.11 g
81.68% C
% H = g H x 100 =
44.11 g
18.32% H

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. 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

7. Quick Quiz.
Calculate the percent by mass of carbon in cadaverine, C5H14N2, a stinky compound present in rotting meat.
67.4% C
58.8% C
51.7% C
68.2% C
% C = g C x 100 =
g total