1. Empirical & Molecular Formulas
Chapter 8
Empirical & Molecular Formulas

2. Types of Formulas
The molecular formula of a compound indicates the actual number of each atom present in a compound.
An empirical formula of a compound indicates the smallest whole number ratio of the elements of each atom present in a compound.

3. Empirical and Molecular Formulas
Substance
Molecular Formula
Empirical Formula
Benzene
C6H6 CH
Acetylene
C2H2
Glucose
C6H12O6
CH2O
Water
H2O

4. Chemical Formulas of Compounds
Chemical formulas give the relative numbers of atoms or moles of each element in a compound - always a whole number ratio.
Example
CO atoms of O for every 1 atom of C or
2 moles of O for every 1 mole of C
If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.

5. To Calculate an Empirical Formula
1. Determine the mass in grams of each element present, if necessary.
2. Calculate the number of moles of each element.
3. Divide each answer from step 2 by the smallest number of moles to obtain the simplest whole number ratio.
If whole numbers are not obtained* in step 3), multiply through by the smallest number that will give all whole numbers
* Be careful! Do not round off numbers prematurely

6. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.

7. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.
2.34g N
5.34g O

8. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.
2.34g N
mol
= mol N
14.0g
5.34g O
mol
= mol O
16.0g

9. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.
2.34g N
mol
= mol N
14.0g
N0.167O0.334
5.34g O
mol
= mol O
16.0g

10. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.
2.34g N
mol
= mol N
14.0g
N0.167O0.334
5.34g O
mol
= mol O
16.0g
0.167 mol N
0.334 mol O
= 1 mol N
= 2 mol O
0.167
0.167

11. A sample of a brown gas, a major air pollutant, is found to contain 2
A sample of a brown gas, a major air pollutant, is found to contain 2.34g N and 5.34g O. Determine a formula for this substance.
2.34g N
mol
= mol N
14.0g
N0.167O0.334
5.34g O
mol
= mol O
16.0g
0.167 mol N
0.334 mol O
= 1 mol N
= 2 mol O
0.167
0.167
NO2

12. To Calculate a Molecular Formula
1. Determine the empirical formula and calculate its mass.
2. Divide the molecular mass by the mass of the empirical formula you determined in step 1. Your answer should be rounded to the nearest whole number.
3. Multiply the empirical formula by the whole number answer found in step 2.

13. A colorless liquid, used in rocket engines has a molar mass of 92
A colorless liquid, used in rocket engines has a molar mass of 92.0 g/mol. What is the molecular formula of the compound if the empirical formula of NO2?

14. A colorless liquid, used in rocket engines has a molar mass of 92
A colorless liquid, used in rocket engines has a molar mass of 92.0 g/mol. What is the molecular formula of the compound if the empirical formula of NO2?
NO2 = (16.0) = 46.0 g/mol

15. NO2 = 14.0 + 2(16.0) = 46.0 g/mol 92.0 g/mol = 2 46.0 g/mol
A colorless liquid, used in rocket engines has a molar mass of 92.0 g/mol. What is the molecular formula of the compound if the empirical formula of NO2?
NO2 = (16.0) = 46.0 g/mol
92.0 g/mol
= 2
46.0 g/mol

16. NO2 = 14.0 + 2(16.0) = 46.0 g/mol 92.0 g/mol = 2 46.0 g/mol
A colorless liquid, used in rocket engines has a molar mass of 92.0 g/mol. What is the molecular formula of the compound if the empirical formula of NO2?
NO2 = (16.0) = 46.0 g/mol
92.0 g/mol
= 2
46.0 g/mol
2 x NO2 = N2O4

17. What is the empirical formula of a compound that contains 53
What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?

18. What is the empirical formula of a compound that contains 53
What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?
53.73g Fe
46.27g S

19. What is the empirical formula of a compound that contains 53
What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?
53.73g Fe
mol
46.27g S
mol
= mol Fe
= 1.44 mol S
55.8g
32.1g

20. What is the empirical formula of a compound that contains 53
What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?
53.73g Fe
mol
46.27g S
mol
= mol Fe
= 1.44 mol S
55.8g
32.1g
0.963 mol Fe
1.44 mol S
= 1 Fe
= 1.5 S
0.963 mol
0.963 mol

21. What is the empirical formula of a compound that contains 53
What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?
53.73g Fe
mol
46.27g S
mol
= mol Fe
= 1.44 mol S
55.8g
32.1g
0.963 mol Fe
1.44 mol S
= 1 Fe
= 1.5 S
0.963 mol
0.963 mol
FeS1.5 x 2 =
Fe2S3

22. What is the molecular formula of a substance with the empirical formula Fe2S3 and a gram formula mass of 624 g/mol?

23. What is the molecular formula of a substance with the empirical formula Fe2S3 and a gram formula mass of 624 g/mol?
Fe6S9

24. 3 x Fe2S3 = Fe6S9 2Fe + 3S = 2(55.8) + 3(32.1) = 207.9 g/mol 624 g/mol
What is the molecular formula of a substance with the empirical formula Fe2S3 and a gram formula molecular mass of 624 g/mol?
2Fe + 3S = 2(55.8) + 3(32.1) = g/mol
624 g/mol
207.9 g/mol
3 x Fe2S3 =
Fe6S9
≈ 3

25. What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?

26. What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?
75.7g C
8.80g H
15.5g O

27. What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?
75.7g C mol C
= 6.31 mol C
12.0g C
8.80g H mol H
= 8.80 mol H
1.0 g H
15.5g O mol O
= mol O
16.0g O

28. What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?
75.7g C mol C
6.31 mol C
= 6.31 mol C
= 6.5 C
12.0g C
0.969 mol
8.80g H mol H
8.80 mol H
= 9 H
= 8.80 mol H
1.0 g H
0.969 mol
15.5g O mol O
0.969 mol H
= 1 O
= mol O
16.0g O
0.969 mol

29. What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?
75.7g C mol C
6.31 mol C
= 6.31 mol C
= 6.5 C
12.0g C
0.969 mol
8.80g H mol H
8.80 mol H
= 9 H
= 8.80 mol H
1.0 g H
0.969 mol
15.5g O mol O
0.969 mol H
= 1 O
= mol O
16.0g O
0.969 mol
C6.5H9O x 2 = C13H18O2

30. Therefore the molecular formula is also C13H18O2
What are the empirical and molecular formulas for Ibuprofen if the molar mass is 206 g/mole and the percent composition of 75.7% C, 8.80% H, and 15.5% O?
C13H18O2
13(12.0) + 18(1.0) + 2(16.0) = 206g/mol
Therefore the molecular formula is also C13H18O2

31. Homework
Empirical & Molecular Formulas Worksheet