1. Empirical Formulas Different than chemical formulas Ethylene:
Propylene:
Reduce all subscripts to their lowest terms. What do you notice about the empirical formula of both compounds?
So what’s the point of empirical formulas if they aren’t the actual chemical formulas found in nature?

2. Determination of an Empirical Formula
Procedure
1.)
2.)
3.)
4.)
Usually helpful to organize the calculations in a table with the following headings:
element
grams
moles
Mole ratio
Formula ratio
Empirical formula

3. Example
A sample of ethylene shows that it contains 85.7 g of carbon and 14.3 g of hydrogen. Determine the empirical formula of ethylene.

4. Example #2
A sample of a pure compound is found to be made up of 1.61 g of phosphorus and 2.98 g of fluorine. Find the empirical formula of the compound.

5. Example #3
An unknown compound is found in the lab to contain 91.8 g of Si and 8.2 g of H. Determine the empirical forumla.

6. Example #3 cont’d
Another experiment indicates that the molar mass of the unknown compound is 122 g/mol. Determine the molecular formula of the compound.

7. How to find the molecular formula of a compound
1.) determine the empirical formula
2.)
3.) determine the molar mass of the compound (usually given)
4.) divide the molar mass of the compound by the molar mass of the empirical formula unit to get “n”, the number of empirical formula units per molecule
5.) Write the molecular formula

8. Using % composition to determine empirical & molecular formulas
A compound contains 52.2% C, 13.0% H, and 34.8% O. Calculate the empirical formula of the compound.