1. Empirical and molecular formulas
C-notes on page 62

2. Finding Empirical Formulas
© 2009, Prentice-Hall, Inc.

3. Calculating Empirical Formulas
One can calculate the empirical formula from the percent composition.
© 2009, Prentice-Hall, Inc.

4. Calculating Empirical Formulas
The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA.
© 2009, Prentice-Hall, Inc.

5. Calculating Empirical Formulas
Step #1: Always assume you have g of the substance, in this case, para-aminobenzoic acid, divide each mass by the molar mass of the element.
C: g x = mol C
H: g x = mol H
N: g x = mol N
O: g x = mol O
1 mol
12.01 g
14.01 g
1.01 g
16.00 g
© 2009, Prentice-Hall, Inc.

6. Calculating Empirical Formulas
Step #2: Calculate the mole ratio by dividing by the smallest number of moles:
C: =  7
H: =  7
N: = 1.000
O: =  2
5.105 mol
mol
5.09 mol
1.458 mol
© 2009, Prentice-Hall, Inc.

7. Calculating Empirical Formulas
These are the subscripts for the empirical formula:
C7H7NO2
© 2009, Prentice-Hall, Inc.

8. Molecular formula
Once you have calculated the empirical formula, find the molar mass of the empirical formula.
To find the molecular formula, you must be given the molecular weight of the compound.
Use the following formula:
whole number ratio= molecular weight
empirical formula weight
* Multiply whole number ratio by each part of your empirical formula

9. Example: These are the subscripts for the empirical formula: C7H7NO2
The molar mass of PABA is g/mol
Find the molecular formula
Find the molecular weight of the empirical formula (138.09)
Use the formula: the whole number ratio= 1
Multiply the empirical formula by ratio
Molecular formula is C7H7NO2
In this example they are the same but not always!!!!!