1. Chapter 7 Chemical Quantities
7.5 Molecular Formulas
Learning Goal Determine the molecular formula of a substance from the empirical formula and molar mass.

2. Molecular Formula
An empirical formula represents the lowest whole-number ratio of atoms in a compound.
A molecular formula represents the actual number of atoms in a compound.

3. Molecular – Empirical Formula

4. Molar Mass, Empirical Formula CH2O

5. Relating the Empirical, Molecular Formula
Once we determine the empirical formula, we can
calculate its mass in grams. Given the molar mass
of the compound, we can calculate the value of the
small integer.

6. Relating the Empirical, Molecular Formula
For example, when the molar mass of ribose is
divided by the empirical formula mass, the integer
is 5.

7. Relating the Empirical, Molecular Formula
Multiplying the subscripts in the empirical formula (CH2O) by 5 gives the formula of ribose, C5H10O5.

8. Calculating a Molecular Formula
Given the empirical formula of ascorbic acid,
Vitamin C, C3H4O3, and the molar mass of the
molecular formula ( g/mol), we can
calculate the molecular formula.

9. Calculating a Molecular Formula
Given the empirical formula of ascorbic acid,
Vitamin C, C3H4O3, and the molar mass of the
molecular formula ( g/mol), we can
calculate the molecular formula.

10. Calculating a Molecular Formula
Given the empirical formula of ascorbic acid,
Vitamin C, C3H4O3, and the molar mass of the
molecular formula ( g/mol), we can
calculate the molecular formula.
Multiplying the subscripts in the empirical formula by 2 gives the molecular formula

11. Guide to Calculating a Molecular Formula from an Empirical Formula

12. Learning Check
A compound is 27.4% S, 12.0% N, and 60.6%
Cl. If the compound has a molar mass of 351 g,
what is the molecular formula?

13. Solution
A compound is 27.4% S, 12.0% N, and 60.6%
Cl. If the compound has a molar mass of 351 g,
what is the molecular formula?
Step 1 Obtain the empirical formula and calculate the empirical formula mass. Write each percentage as the number of grams in a 100. g sample.
S = 27.4 g N = 12.0 g Cl = 60.6 g

14. Solution
Step 1 Obtain the empirical formula and calculate the empirical formula mass.

15. Solution
Step 1 Obtain the empirical formula and calculate the empirical formula mass.
Obtain the subscripts of each element.

16. Learning Check
Step 1 Obtain the empirical formula and calculate the empirical formula mass.

17. Learning Check
Step 2 Divide the molar mass by the empirical formula mass to obtain a small integer.

18. Learning Check
Step 3 Multiply the empirical formula by the small integer to obtain the molecular formula.
Multiplying the subscripts in the empirical formula by 3 gives the molecular formula.

19. Concept Map