1. 7.3 Molecular Formulas
3/8/2016

2. Empirical Vs. Molecular Formulas
A molecular formula tells exactly how many of each atom are in the molecule. C O H
An empirical formula tells the ratio of atoms in a molecule C H O
empirical formula of sugar = CH2O
molecular formula of sugar = C6H12O6

3. A molecular formula is a whole-number (n) multiple of the empirical formula.
Compound
Empirical Formula
Molecular formula
Molar Mass (grams/mole)
Formaldehyde
CH2O
30.03
Acetic acid
C2H4O2
n = 2
60.06
Glucose
C6H12O6
n = 6
180.18

4. Example: The empirical formula for a compound is CH2O
Example: The empirical formula for a compound is CH2O. Its experimental molar mass is g/mol. Determine the molecular formula of the compound.
1st Find the molar mass of the empirical formula CH2O.
1 x g = g/mol
2 x 1.01 g = 2.02 g/mol
+ 1 x g = g/mol
molar mass of CH2O = g/mol

5. 2nd Solve for n, the factor multiplying the empirical formula n = experimental molar mass of compound molar mass of empirical formula n = g/ mol = g/mol Molecular Formula = n(E.F) 4(CH2O) =
C4H8O4

6. Example: Isobutylene is a raw material for making synthetic rubber
Example: Isobutylene is a raw material for making synthetic rubber. It contains 85.7% carbon and 14.3% hydrogen by mass. The experimental molar mass is g/mol. What are the empirical and molecular formulas of isobutylene?

7. 85.7 g C / 12.01 g C = 14.3 g H / 1.01 g H = 7.13 mol C 14.2 mol H
Find E.F first….
85.7 g C / g C =
14.3 g H / 1.01 g H =
7.13 mol C
14.2 mol H
C H 14.2
CH2

8. Find n for M.F
1 x g C =
2 x 1.01 g H =
12.01 g C
2.02 g H
14.03 g/mol CH2
n= g/ mol = 4
14.03 g/ mol
4(CH2) 
C4H8

9. Empirical formula = CH2 Molecular formula = C4H8