The Molecular Formula tells you the number
of atoms of each of the elements in one molecule
of a compound.
In a previous video, we found the “empirical
formula” and this is a related idea, but
they are not the same thing.
The Empirical Formula gives the lowest whole
number ratio of the atoms of the elements
in a compound.
This may or may not be the same as the molecular
formula for a given compound.
The molecular formula is sometimes called
the “true formula” because it tells you
how many of each kind of atom are actually
present in the molecule.
It’s important to note that in a molecule,
the arrangements of the atoms in 3 dimensions
makes a big difference.
Neither the molecular formula nor the empirical
formula spell out what the molecule looks
like in three dimensions.
For that, we need a structural formula, and
we’ll save that complicated idea for another video
So what IS the relationship between these
two kinds of chemical formulas?
The molecular formula is either the same as
the empirical formula or it is a simple whole-number
multiple of the empirical formula.
It’s going to have the same ratio of atoms
as the empirical formula, but the actual number
of atoms may be different.
For example, formaldehyde and acetic acid
have the same empirical formula, CH2O, but
different molecular formulas.
Formaldehyde is an interesting case, because
its molecular formula is exactly the same
as its empirical formula.
This tells you the actual number of each kind
of atom in each molecule of formaldehyde :
1 atom of carbon, 2 atoms of hydrogen, and 1
atom of oxygen.
By contrast, the molecular formula of acetic
acid is a multiple of its empirical formula.
If you multiply the empirical formula by 2,
you get the molecular formula of acetic acid:
C2H4O2.
Notice the ratio of atoms is the same.
But in each molecule of acetic acid, there
are actually
2 atoms of Carbon, 4 atoms of Hydrogen, and 2 atoms of Oxygen.
To find the molecular formula of a compound,
you need to have its empirical formula and
its molar mass.
The goal is to find out how many empirical
formula units are in one molecule of the compound.
Our strategy is to
Calculate the empirical formula mass
Divide the molar mass by the empirical formula
mass.
This tells you how many of the empirical formula
units are needed to form
one molecule of the  compound.
This should be a whole number answer, or very
close.
If your answer isn’t an integer or very
close to one, you’ve made a mistake somewhere.
Multiply the empirical formula by this value.
Let’s see some examples:
Example 1: A compound has the empirical formula
CH2O.
It has a molar mass of 180 g/mol.
What is the molecular formula?
This empirical formula looks familiar - it’s
the same empirical formula as formaldehyde
and acetic acid, as we talked about earlier.
Let’s go through the steps and see if it’s
one of those.
STEP 1: Calculate the empirical formula mass.
We get that from the periodic table.
12.011 + 2(1.008) + 15.999 = 30.026 g/mol
STEP 2: Divide the molar mass by the empirical
formula mass.
180 g/mol / 30.026 g/mol = 5.99 We’re looking
for a simple whole number to multiply the
empirical formula by, so we’ll round to
6.
STEP 3: Multiply the empirical formula by
this value: 6 times (CH2O) = C6H12O6.
This is glucose.
So now we know 3 compounds that have the same
empirical formula,
but different molecularformulas.
Example 2: For our next example, let’s work
a problem all the way through from the percent
composition data and the molar mass to get
the molecular formula.
This means first we will find the empirical
formula, and then go on to find the molecular
formula.
Our experimental data tells us the compound
is 94.1% O and 5.9% H. The molar mass is 34g/mol.
First, we’ll find the empirical formula,
which only requires the % composition data.
Assume 100g.
Then we have 94.1g Oxygen and 5.9g Hydrogen.
Convert to moles to get the mole ratio.
For oxygen, we have 94.1g (1 mol/15.999g)
= 5.88 mol oxygen
For hydrogen, we have 5.9g (1 mol/1.008g)= 5.85 mol 
Hydrogen
H 5.85 O 5.88 Divide through by the lowest
number, 5.85.
We get
H1O1 (remember the 1s are understood, so we
get HO as the empirical formula.
Next we find the empirical formula mass.
1.008 g/mol + 15.999 g/mol = 17.007 g/mol
Now we use the molar mass given in the problem.
Molar mass/ empirical formula mass = 34.0
g/mol / 17.007 g/mol = 2
In other words, there are 2 empirical formula
units per 1 of these molecules.
So we multiply the empirical formula by 2:
2(HO) = H2O2 hydrogen peroxide.
What if you see a problem asking you to go
in the other direction?
Given a molecular formula, what is the empirical
formula?
Example 3: Butane has the molecular formula
C4H10.
What is the empirical formula?
Notice there is no other information given
in the problem.
We don’t need any percent composition data
or molar mass information to solve this problem.
The empirical formula is going to be the molecular
formula divided by the Greatest Common Divisor
of the subscripts.
The subscripts here are 4 and 10.
If you factor these, 4 is 2x2, and 10 is 2x
5.
The greatest common divisor is 2, so we will
divide through by 2 to simplify the molecular
formula down to the empirical formula.
C4/2 H10/2 = C2H5.
Remember, an empirical formula may or may
not be the same as the molecular formula.
The molecular formula is always a simple multiple
of the empirical formula.
For example, for the empirical formula CH2O,
there are many possible multiples that correspond
to different compounds.
CH2O is both the empirical formula and molecular
formula for formaldehyde, but if we multiply
that empirical formula by 2, we get C2H4O2,
which is the molecular formula for acetic acid.
If we multiply CH2O by 6, we get C6H12O6,
which is glucose.
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