2. Percent Composition
• The percent composition is the relative amounts of the elements in a
compound.
• The percent composition of a compound can be determined when given the
formula of a compound.
Here we can compare the mass of each element present in 1 mole of
the compound by the total mass of 1 mole of the compound and then
multiply it by 100 to convert into a percent.
3. Example 1: K(ClO3)
% Composition of an Element =
K 1 x 39.10 = 39.10 %K = 39.10 g x 100 = 31.91 %K
Cl 1 x 35.45 = 35.45 122.55 g/mol
O 3 x 16.00 = 48.00
122.55 g/mol %Cl = 35.45 g x 100 = 28.93 %Cl
122.55 g/mol
%O = 48.00 g x 100 = 39.17 %O
122.55 g/mol
total mass of element in compound
X 100-----------------------------------------------------
total mass of compound (molar mass)
4. Example 2:
A compound is formed when 9.03 g Mg combines with 3.48 g N. What
is the percent composition of this compound?
% Composition of an Element =
Mg = 9.03 g %Mg = 9.03 g x 100 = 72.18 %Mg
N = 3.48 g 12.51 g
12.51
%N = 3.48 g x 100 = 27.82 %N
12.51 g
total mass of element in compound
X 100-----------------------------------------------------
total mass of compound (molar mass)
5. Example 3:
When a 14.2 g sample of mercury (II) oxide is decomposed into its
elements by heating, 13.2 g of Hg is obtained. What is the percent
composition of the compound?
% Composition of an Element =
14.2 g %Hg = 13.2 g x 100 = 92.96 %Hg
Hg = 13.2 g 14.2 g
O = 1.0 g
%O = 1.0 g x 100 = 7.04 %O
14.2 g
total mass of element in compound
X 100-----------------------------------------------------
total mass of compound (molar mass)
6. Example 4:
A sample of unknown composition with a mass of 0.562 g has the
following composition: 13.0% carbon, 2.20% hydrogen, and 84.8%
fluorine. When this compound is decomposed into its elements, what
mass of each element would be recovered?
% Composition of an Element =
Mass of C = (13.0%)(0.562g) = 0.0731 g C
100
Mass of H = (2.20%)(0.562g) = 0.0124 g C
100
Mass of F = (84.8%)(0.562g) = 0.477 g C
100
(% composition)(mass of sample)
-----------------------------------------------------
100
