Empirical and Molecular Formulas
Law of Multiple Proportions
 If A and B react to form compounds, the different masses of B that combine with
a fixed mass of A can be expressed as whole number ratios.
 If we know the percent composition of the elements that make up different
compounds, we can determine the empirical formula for each compound
Empirical Formula
-
The simplest whole number formula for a compound
For ionic compounds, this simply corresponds to the formula unit
For molecular compounds, this may or not be the same as the molecular formula
It therefore describes the relative ratio between the elements in the molecule
The empirical formula of a molecule can be determined by analyzing the percent composition
data for the compound. The following steps should be followed when solving these types of
problems.
1. Assume a 100 g sample of the compound.
2. Using the percent values, convert this to mass values
3. Determine the number of moles of each element in the compound, by taking the mass
values from 2 and dividing by the molar mass of the element.
4. Divide each molar value by the lowest molar amount to get the whole-number ratio of
each element in the compound.
Example 1: Calculate the empirical formual of a compound that is 85.6% carbon and 14.4%
hydrogen.
1. Assume a 100 g sample.
2. Therefore, the mass of carbon is 64.3 g, the mass of hydrogen is 7.2 g and the mass of
oxygen is 28.5 g.
3. Set up a table
Element
C
H
Grams per
100g
85.6
14.4
Molar Mass
(g/mol)
12
1
Number of moles
(n = g/M)
7.13
14.3
Molar amount  lowest
molar amount
7.13  7.13 = 1
14.3  7.13 = 2.01
Therefore, the empirical formula expressed using the values above would be: C7.13H14.3
However, an empirical formula is the simplest whole-number ratio of each element in the
compound. So we…
4. Divide each molar amount by the lowest whole-number
Carbon has the lowest molar amount, therefore divide each by 7.13
which gives… CH2
Example 2: Acrolein is an organic compound that consists of 64.3% carbon, 7.2% hydrogen and
28.5% oxygen by mass.
Element
C
H
O
Grams per
100g
64.3
7.2
28.5
Molar Mass
(g/mol)
12
1
16
Number of moles
(n = g/M)
5.36
7.2
1.78
Molar amount 
lowest molar amount
5.36  1.78 = 3
7.2  1.78 = 4
1.78  1.78 = 1
Therefore, the empirical formula expressed using the values above would be : C3H4O
Molecular Formula
- Describes the molecule as a whole
- The subscripts show the actual number of atoms of each element in the molecule
The molecular formula of water is H20, and its empirical formula is H20.
The molecular formula for benzene is C6H6 and its empirical formula is CH.
The molecular formula for glucose is C6H12O6, and its empirical formula is CH2O.
How can we determine the molecular formula? If the empirical formula and the molar mass
are given, you can determine the molecular formula as well:
1. Determine the molar mass of the empirical formula
2. Divide the molar mass of the molecular formula by the molar mass of the empirical
formula.
3. Multiply the coefficients by the number obtained in 2.
The molar mass of acrolein is 56.1 g/mol.
1) Empirical Molar Mass of C3H4O =
2) Divide the molar masses
3) Multiply the empirical formula subscripts by the coefficient.
Therefore, the molecular formula of acrolein is _______________