Empirical and Molecular Formula
Empirical Formula: shows the smallest whole-number ratio for the elements in a compound.
Ex. C
4
H
8
 empirical formula: ______
Determine empirical formula using percentage composition of a compound:
Ex. 1: The percentage composition of a compound is 50.81% zinc, 16.04% phosphorous, and 33.15% oxygen
Step 1: List the elements
Step 2: Calculate the mass of each element
Step 3: Convert to moles
Step 4: Determine the ratio
Step 5: determine the whole number ratio
Element Mass Number of
Moles
Ratio of
Moles
Whole
Number Ratio
Empirical Formula:
Ex. 2: Determine the empirical formula for a compound that contains 69.88% iron and 30.21% oxygen.
Element Mass Number of
Moles
Ratio of
Moles
Whole
Number Ratio
Empirical Formula:

Molecular Formula: gives the exact number of atoms of each type of element
Ex. C
6
H
6
The molecular formula is sometimes the same as the empirical formula
Ex. CO
2
To determine the molecular formula you need the empirical formula and the molar mass
Step 1: Determine the empirical formula
Step 2: Determine the ratio between the empirical formula mass and molar mass
Step 3: Apply the ratio to the empirical formula and determine the molecular formula
Ex. 1: A compound with the empirical formula CH has a molar mass of 78g/mol. what is the empirical formula?
Ex. 2: A chemical analysis indicates that a compound is 28.64% sulfur and 71.36% bromine. The molar mass is 223.94 g/mol. What is the molecular formula?