1. Empirical Formula of a Compound

2. Molecular Formula (MF)
actual formula
shows the number of atoms of each element that make up a molecule or a formula unit

3. Empirical Formula (EF)
simplest formula of a compound
shows the lowest whole number ratio of the elements in a compound
can be determined from
- the molecular formula or
- percentage composition of the
compound

4. Using Molecular Formula - Examples
Substance
Molecular Formula
(MF)
Empirical Formula
(EF)
Butane
C4H10
C2H5
Glucose
C6H12O6
Benzene
C6H6
Water
H2O

5. Using Molecular Formula
- Examples
Substance
Molecular Formula
(MF)
Empirical Formula
(EF)
Butane
C4H10
C2H5
Glucose
C6H12O6
CH2O
Benzene
C6H6 CH
Water
H2O

6. Using Percentage Composition - Examples
Calculate the EF of a compound that is 85.6 % carbon and 14.4 % hydrogen.
Answer:
Assume that you are using a 100 g sample of the compound and complete the next calculations. Use a table for your data.

7. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C H

8. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01 H
14.4
1.01
Recall: n = m M

9. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01
7.13 H
14.4
1.01
14.3
Recall: n = m M

10. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01
7.13 H
14.4
1.01
14.3
Calculate LWN ratio by dividing: molar amount
lowest molar amount

11. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01
7.13 H
14.4
1.01
14.3
Calculate LWN ratio by dividing: molar amount
lowest molar amount

12. Using Percentage Composition - Example #1
Lowest Whole Number
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01
7.13
7.13 = 1.00 H
14.4
1.01
14.3
14.3 = 2.01
Calculate LWN ratio by dividing: molar amount
lowest molar amount

13. Using Percentage Composition - Example #1
Element
Mass %
m (g)
Molar Mass
M (g/mol)
Moles
n (mol)
LWN
Ratio EF C
85.6
12.01
7.13
7.13 = 1.00 H
14.4
1.01
14.3
14.3 = 2.01 H2
Empirical Formula: CH2

14. Using Percentage Composition - Examples
2. Calculate the EF of a compound with the following % composition: 32.4 % Na, 22.6 % S and 45.0 % O 3. Calculate the EF of TNT, with the following % composition: 37.0 % C, 2.20 % H, 18.5% N and 42.3 % O Hint: see table 6.2 (p210)