Empirical & Molecular Formulas
Chapter 3
Mathematical Methods
STEP 1
Obtain in the laboratory, the number of grams OR the weight percentage of each element in the
compound. This can be found by:
- breaking down the existing compound into its elements
building the compound from the elements
STEP 2
Determine the number of moles of each element in the compound.
Use dimensional analysis with the atomic mass of the element.
STEP 3
Find the simplest whole number ratio of the moles of each element. Since each mole contains
the same number of atoms, the simplest whole number ratio of the moles is also the simplest whole
number ratio of the atoms of each element.
To do this, take all the mole values and divide them by the SMALLEST one
The answers are the subscripts in the empirical formula
.05 Rule
After step #3, if the value is within .05 of a whole number (+ 0.05 or - 0.05), then the value may be
rounded to that whole number.
The values used in these problems are obtained by experimentation. The 0.05 rule allows for
experimental error.
IF the application of the .05 Rule does not produce whole numbers, then ALL of the results of step 3
must be multiplied by the same smallest integer that WILL produce values that can be rounded to
whole numbers by the .05 Rule.
Empirical Formula Example:
A compound is found to contain 72.3% Fe and 27.7% O by weight. Calculate the empirical formula.
Example of a hydrated compound
CaSO4  7 H2O
Compounds with molecules of water held in their crystal structure
Water can be removed by heating, leaving behind what is called the anhydrous compound
Naming -- the following is tacked on the name obtained from the ions
H2O
monohydrate
2 H2O
dihydrate
3 H2O
trihydrate
4 H2O
tetrahydrate
5 H2O
pentahydrate
6 H2O
hexahydrate
7 H2O
heptahydrate
8 H2O
octahydrate
9 H2O
nonahydrate
10 H2O
dekahydrate
CaSO4  7 H2O -- named as calcium sulfate heptahydrate
Empirical Formula of a Hydrate Example:
Find the empirical formula of a hydrate of CaSO4 hydrate that is 28.5% H2O
A molecular formula tells the actual number of atoms of each element in a molecule.
It is a multiple of the empirical formula.
Molecular Formula Example:
The compound with a molar mass of 171.0 g/mole that contains 14.0% carbon, 41.5% chlorine, and
44.4% fluorine is a chlorofluorocarbon (CFC) that was once used as a refrigerant, but is now on the
list of chemicals known to be ozone depleting. What is the molecular formula of this compound?
Combustion Analysis Example:
A 3.489g sample of a compound containing C, H, and O yields 7.832 g of CO2, and 1.922g of water
upon combustion. What is the simplest formula of the compound?
Since it’s combustion, we know:
CxHyOz + O2  CO2 + H2O
All the C goes into the CO2
All the H goes into the H2O
(our job is to find x, y, and z!)