Percentage
Composition
What is percentage composition?
The mass of each element in a
compound compared to the entire
mass of the compound and
multiplied by 100 percent is called
the percentage composition of the
compound.
To calculate percentage
composition:
Mass of element
X
Total mass of compound
100
=
%
The percentage composition of a
compound tells you the percent of
the mass made up by each
element of the compound. This
can be determined by calculating
the molar mass of a given
chemical formula.
Calculating percentage
composition of a compound:
EX: What is the percentage composition of hydrogen
and oxygen in H2O?
Formula Mass =
H= 2 atoms X 1 amu= 2 amu
O= 1 atom X 16 amu= 16 amu
18amu
Percentage of Hydrogen =
Percentage of Oxygen =
2.0 g H2
18 g H2O X 100 = 11%
16g O
18g H20 X 100 = 89%
(CHECK: Percentages should equal 100%.)
Calculating mass of an element
in a compound:
EX:
How many grams of oxygen are in 5 g of H2O?
__X__
=
16g O
5 g H 20
18gH20
(Cross Multiply)
18 x = (16) (5)
x = 80
18
x = 4.44 g O
PRACTICE PROBLEMS
KNO3
H2SO4
C2H5OH
C6H5NH2
What is the percentage of carbon in
CO2?
How many grams of carbon are in 25 g
of CO2?
Find the percentage composition of a
compound that contains 2.63g of
carbon, 0.370g of hydrogen, and
0.580g of oxygen in a 3.58g sample
of the compound.