Name: ____________________________________________________ Period ______ Date __________________
7.4 Practice Worksheet
Determining Empirical & Molecular Formulas
Part 1: Empirical vs. Molecular Formula
When scientists write the smallest ratio of atoms in a compound, this is called an empirical formula.
However, this can be different than the formula that indicates the exact number of atoms of each element
in the compound, which is called a ____________________ formula. Note: the empirical and molecular
formulas for ionic compounds are always the same.


Smallest ratio atoms = ________________ formula
Actual number of atoms or ions present = _________________ formula
Circle the empirical formula:
CH2
or
C2H4 (ethene)
CH2O
or
C6H12O6 (glucose)
H2O
CaO
or
or
H2O
CaO
Part 2: Determining an Empirical Formula
The empirical formula of a compound must be determined ______________________ .
Here’s a little rhyme to help you remember how to solve these…
Step 1: _______________________ Step 2: _____________________

Step 3: ____________________
Tip: Always assume you are working with a 100 g starting sample, if the actual masses of each
element are not provided.
1. A compound was found to be made of 60.0% C, 13.4% H, and 26.6% O. What is the empirical
formula of this compound?
2. Find the empirical formula for a sample containing 53.73 g of Fe and 46.27 g of S:
3. Find the empirical formula for a compound that contains 15.77% Al, 28.11 % S, and 56.12% O:
Part 3: Determining a Molecular Formula (true formula)
If you know the empirical formula of a compound and its molecular mass (true molar mass), it is possible to
determine its molecular formula. (Of course, if the empirical formula is not provided, you will have to do
that step first).
1. What is the molecular formula of the compound with empirical formula C3H8O and a molecular mass
of 180.33 g/mol?
2. A compound is found to be 40.0% carbon, 6.70 % hydrogen, and 53.5% oxygen. Its molecular mass is
60.0 g/mol . What is its molecular formula?
3. A compound is 64.9% C, 13.5% H, and 21.6% O . Its molecular mass is 74.0 g/mol . What is its
molecular formula?