1. Empirical & Molecular Formula
SCH4C
Ms. Richardson

2. Empirical & Molecular Formula
In Dalton’s atomic theory, he stated that atoms will combine with one another in simple, whole number ratios to form compounds.
An example of a molecular formula (the actual formula) is C6H6. This is Benzene.
An example of a simple whole number ratio, or the empirical formula for Benzene would be CH.

3. Comparing molecular and empirical formulas
Name of Compound
Molecular Formula
Empirical Formula
hydrogen peroxide
H2O2 HO
glucose
C6H12O6
CH2O
benzene
C6H6 CH
ethyne
C2H2
water
H2O

4. Determining Empirical Formula
You already know how to calculate percent composition.
In this section, you will use percent composition to find the formula of the compound.
These are the 3 steps to determining empirical formula.
Determine the mass of the elements present.
Determine the moles of the elements present.
Divide the # of moles of each element by the lowest number of moles

5. Empirical formula example
What is the empirical formula of a compound with 79.9 g of Cu and g of S?
“Percent to mass
Mass to mole
Divide by small
Multiply ‘til whole”

6. Another example
Calculate the empirical formula of a compound that has 85.6% carbon and 14.4% hydrogen.
HINT: Assume that the percentage composition is based on 100g of the compound.

7. A tricky Example
What happens when the mole ratio does not give you nice whole numbers?
The percent composition of a fuel is 81.7% carbon and 18.3% hydrogen. What is the empirical formula?

8. Getting to whole numbers
0.2, 1.2, 2.2, 3.2……. Multiply by 5
0.25, 1.25, 2.25 … …Multiply by 4
0.3, 1.3, 2.3, 3.3…… Multiply by 3
0.5, 1.5, 2.5, 3.5…… Multiply by 2

9. Molecular Formula
Once we have determined the empirical formula (the smallest whole number ratio), we can determine the compound’s actual formula:
Determine the molar mass of the empirical formula
Divide the given molar mass by the molar mass of the empirical formula.