# Unit: Chemical Quantities Determining the Molecular Formula from ```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
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
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