Unit: Chemical Quantities
Determining the Molecular Formula
from the Empirical Formula
After today, you will be able to…
• Calculate the empirical formulas
for various compounds given their
percent makeup
Molecular Formula Determination
from the Empirical Formula
To determine molecular formulas from the
empirical formula follow these steps:
1. First determine the empirical formula
2. Use the formula:
Molar mass of molecular formula
Molar mass of empirical formula
3. Multiply the subscripts in the empirical
formula by this whole number to get the
molecular formula
Example: What is the molecular formula for
a compound that is 74.0% C, 8.70% H, and
17.3% N and has a molar mass of 243.39g?
70.4gC x 1 mol C 6.16
Empirical: C5H7N
=
= 5
12.01gC 1.23
5C=12.01(5) = 60.05
7H=1.01(7) = 7.07
8.70gH x 1 mol H = 8.61
= 7 1N=14.01(1) = +14.01
1.01gH
1.23
81.07
17.3gN x 1 mol N = 1.23 = 1
14.01gN 1.23
243.39
81.07
= 3 x C5H7N
C15H21N3
Time to Practice!
Complete WS 7