1. Empirical Formulas
Unit 5

2. Percent Composition
Percent composition is a number that tells us how much of an element makes up a compound
The formula used to calculate percent composition is:

3. Sample Problem
An g sample of H2O contains 2.02 g of H and g of O. What is the percent by mass for each element?
11.2 % H
88.80 % O

4. Calculating Percent Composition
Step #1: Calculate the molar mass of the compound
Step #2: Use the atomic mass of each element and the formula for percent composition to calculate the percent composition of each element
Example: NaHCO3

5. Empirical Formula
If we know the percent composition for each element of a compound, we can calculate the compound's empirical formula
An empirical formula is the formula with the smallest whole number ratio of the elements

6. Calculating Empirical Formula
Step #1: Convert grams of each element to moles
Step #2: Divide each of these numbers by the smallest number
Example:
What is the empirical formula for a compound that contains 6.93 g of oxygen and 0.43 g of hydrogen?

7. Sample Problem
A sample of unknown origin contains 48.64% C, 8.16% H, and % oxygen. What is the empirical formula for this compound?
C3H6O2

8. Molecular Formula
A molecular formula tells you exactly the number of atoms present in a molecule of a compound
Molecular formulas can be calculated by using empirical formulas

9. Calculating Molecular Formulas
Molecular formulas can be calculated using the equation below:

10. Sample Problem
The empirical formula for succinic acid is C2H3O2. The molar mass of succinic acid is g/mol. What is the molecular formula for succinic acid?
C4H6O4