Wake-up
1. Determine the empirical formula for the
following: 28.4% Copper and 71.6% Bromine
Molecular Formula
What are Molecular Formulas?
How to calculate molecular formulas?
Empirical Formula
A chemical formula that shows the lowest whole
number ratio of all atoms in a compound
Molecular Formula
A chemical formula that shows the total number
and kinds of atoms in a molecule
Recall from yesterday
Compound A is
Empirical Formula
Compound B is
Molecular Formula
How to find Molecular Formulas?
1. Write down what you know.
The simplest formula for butane is C2H5 and its molecular
mass is 60g/mol. What is the molecular formula of
butane?
Empirical formula = C2H5
Molar Mass = 60g/mol
How to find Molecular Formulas?
2. Calculate/Find the Empirical Formula
(if needed)
The simplest formula for butane is C2H5 and its molecular
mass is 60g/mol. What is the molecular formula of
butane?
C2H5
How to find Molecular Formulas?
3. Calculate the empirical formula mass (efm)
using the empirical formula
C = 2 x 12.01 = 24.02
H = 5 x 1.00 = 5
Empirical Formula Mass = 24.02 + 5
29.02
How to find Molecular Formulas?
4. Divide the molar mass (that is provided in the
problem) by the empirical formula mass
The simplest formula for butane is C2H5 and its molecular
mass is 60g/mol. What is the molecular formula of
butane?
60g/mol
29.02g
=
2 mole
How to find Molecular Formulas?
5. Multiply the empirical formula subscripts by
the whole number you found in step 4 to get the
molecular formula.
C2H5
x
2
=
C4H10
Example #1
The simplest formula for vitamin C is C3H4O3.
Experimental data indicates that the molecular
mass of vitamin C is about 180 g/mol. What is
the molecular formula of vitamin C?
Example #2
A compound contains 18.8% sodium, 29%
chlorine, and 52.2% oxygen, by mass. If the
molar mass of the compound in 122.44 g/mol,
determine the empirical and molecular
formulas.
Example #3
Caffeine has an elemental analysis of 49.48%
carbon, 5.190% hydrogen, 16.47% oxygen, and
28.85% nitrogen. It has a molar mass of 194.19
g/mol. What is the molecular formula of
caffeine?
Molecular Formula
What are Molecular Formulas?
How to calculate molecular formulas?