Percent
Composition of
Compounds
Percent by Mass
Chemical formulas show the ratio of atoms
in a compound.
 Sometimes, it is helpful to know the
composition of a compound based on the
masses of different elements represented.

Calculating Percent Composition
Mass fraction =
mass of element in 1 mol of compound
mass of 1 mol of compound
How do you convert a fraction to a
percentage?
Multiply by 100
Example Problem 1
Ethanol is an alcohol obtained by fermenting
sugar in grapes, corn and other fruits and
grains. It is often added to gasoline as an
octane enhancer to form a fuel called
gasahol. Ethanol’s formula is C2H5OH.
This means each molecule of ethanol
contains 2 C atoms, 6 H atoms, and 1 O
atom. What is the mass percent of each
element present in ethanol?
Example Problem 2
Carvone is a substance that occurs in two
forms, both of which have the same
molecular formula (C10H14O) and molar
mass. One type of carvone gives caraway
seeds their characteristic smell. The other
is responsible for the smell of spearmint
oil. Compute the mass percent of each
element in carvone.
Example Problem 3
A compound is analyzed and determine to
contain 1.75 g of carbon and 46.75 g of
bromine. What is the percent composition
of this compound?
Determining
Empirical Formula
Difference between Empirical
and Molecular Formulas

Empirical formula: expresses the smallest
whole-number ratio of atoms in a
compound
 Ionic
compounds are always written in
empirical formula

Molecular formula: the actual formula of a
compound giving the types of atoms and
numbers of each type of atom.
 Covalent
compounds (molecules) are always
written in molecular formula
Difference between Empirical
and Molecular Formulas
Glucose has a molecular formula of
C6H12O6. We must write its formula this
way to show that a glucose molecule is
actually made up of 6 C atoms, 12 H
atoms, and 6 O atoms. What is the
empirical formula of glucose?
CH2O
Empirical Formula
Finding the empirical formula of a
compound is determining the ratio of
atoms in a compound.
 Determining empirical formula is important
when you know the elements present in a
compound but you do not know what the
compound is.

Steps for Determining the Empirical
Formula of a Compound
1.
2.
3.
Obtain the mass of each element present
(in grams). If percent composition is
given, assume a 100-g sample of the
compound.
Determine the number of moles of each
type of atom present.
Divide the number of moles of each
element by the smallest number of moles
to convert the smallest number to 1.
4.
5.
If the numbers you get in step #3 are
whole numbers they are the subscripts in
the empirical formula. If one or more of
these numbers are not whole numbers,
go on to step 5.
Multiply the numbers from step #3 by the
smallest whole number that will convert
all of them to whole numbers. This set of
numbers will be the subscripts for the
empirical formula.
Example Problem 1
An oxide of aluminum is formed by the
reaction of 4.151 g of aluminum with 3.692
g of oxygen. Calculate the empirical
formula for this compound.
Example Problem 2
When a 0.3546-g sample of vanadium metal
is heated in air, it reacts with oxygen to
reach a final mass of 0.6330 g. Calculate
the empirical formula of this vanadium
oxide.
Example Problem 3
A sample of lead arsenate, an insecticide
used against the potato beetle, contains
1.3813 g of lead, 0.00672 g of hydrogen,
0.4995 g of arsenic, and 0.4267 g of
oxygen. Calculate the empirical formula of
lead arsenate.
Example Problem 4
Cisplatin, the common name for a platinum
compound that is used to treat cancerous
tumors, has the composition (mass
percent) 65.02% platinum, 9.34% nitrogen,
2.02% hydrogen and 23.63% chlorine.
Calculate the empirical formula for
cisplatin.
Determining
Molecular Formula
Molecular Formula
If we know the percent composition of a
compound in terms of the masses of the
elements present we can calculate the
empirical formula but not the molecular
formula.
 In order to calculate the molecular formula
we must know the molar mass.

Molecular Formula
1.
2.
3.
If the empirical formula is not given,
follow the steps calculate the empirical
formula.
Find the empirical formula mass.
Compare the empirical formula mass to
the molar mass.
molecular formula = (empirical formula)n
n = molar mass/empirical formula mass
Example Problem 1
A white powder is analyzed and found to
have an empirical formula of P2O5. The
compound has a molar mass of 283.88
g/mol. What is the compound’s molecular
formula?
Example Problem 2
A compound used as an additive for
gasoline to help prevent engine knock
shows the following percentage
composition: 24.27% C, 4.07% H, and
71.65% Cl. The molar mass is known to
be 98.96 g. Determine the empirical
formula and the molecular formula for this
compound.
Example Problem 3
A certain hydrocarbon (a compound
consisting of carbon and hydrogen) is
82.66% carbon by mass and has a molar
mass of 58.12 g/mol. Determine the
molecular formula of this hydrocarbon.