1. Empirical and Molecular Formulas
April 2016

2. Empirical vs Molecular
Is the simplest (i.e. reduced) whole number ratio of atoms in a compound
All formulas for ionic compounds are empirical
Example: NaCl, not Na2Cl2
Not all correct formulas for covalent compounds are empirical
Example: H2O2
Is the same or a multiple of the empirical formula
It shows the actual number of atoms in a compound
Example: N2O4 is the molecular formula for dinitrogen tetraoxide and it is double its empirical formula NO2

3. Complete the chart below:
Empirical formula
Molecular formula
CO2
H2O2
C3H6
C2H4O2
SnO

4. To determine the empirical formula of a compound:
You need to know the number of moles of each element present in the compound.
Example: What is the empirical formula for the compound that contains:
1 mol S, 4 mol O and 2 mol Na?
b) 4 mol C and 10 mol H
c) 1 mol Cl and 2.5 mol O
d) 0.4 mol C and 1.2 mol H

5. Empirical formulas can be determined from:
A) Mass of elements in the compound Example 1: A sample of caffeine contains g of carbon 2.51 g of hydrogen g of nitrogen 8.02 g of oxygen

6. Example 2: A sample of a compound contains: 56. 4g phosphorus 43
Example 2: A sample of a compound contains: 56.4g phosphorus 43.6g oxygen

7. Empirical formulas can be determined from:
B) Percent composition Example 3: A compound has the following % composition 21.6% sodium 33.3% chlorine 45.1% oxygen

8. Complete p. 102 #1 and 2 showing full solutions in the workbook.

9. Determining Molecular Formulas
You need the empirical formula and the molar mass
Example 1:
The empirical formula of a compound is CH2 and its molar mass is
84.16 g/mol. What is its molecular formula?
Example 2:
The empirical formula for caffeine is C4H5N2O and its molar mass is g/mol. What is its molecular formula?

10. Example 3: A compound has the following % composition 63. 13% carbon 8
Example 3: A compound has the following % composition 63.13% carbon 8.831% hydrogen 28.04% oxygen
a) What is its empirical formula? b) If the molar mass of the compound is g/mol, what is the molecular formula?

11. Example 4: A compound has the following % composition 85.63% carbon 14.37% hydrogen and a molar mass of g/mol, what is the molecular formula?

12. Complete p. 104 #2-4 showing full solutions in the workbook.