Overview: This section provides
a review of chemical formulas and the information that is available from the
different, but equivalent representations (formulas) of a molecule/compound.
First what is a chemical formula? They have been used numerous times already in this tutorial
but so far, they have not been defined.
Chemical Formula: A representation of the
chemical composition of a substance. This can be either a molecular or empirical
Basically a chemical formula gives scientists a variety of information about a particular compound.
Let's look at iso-octane. Iso-octane is the component of gasoline that burns the smoothest. Iso-octane
can be written in numerous ways.
Experimentally, the mass percentage of a compound is obtained by means of combustion
analysis or other types of elemental analysis. We can use mass percentages to determine empirical
formulas, but not molecular formulas.
This means that iso-octane is 84.12% carbon. The percent hydrogen must be 100% - 84.12% since the
total elemental percentages must add up to 100. Therefore hydrogen accounts for 15.88% of the molecular mass.
Notice that we could have found the % H first and then subtracted to solve for the percent carbon. Try solving
for the percent hydrogen first and then the percent carbon to verify that you get the same answer.
Once you have the percent elemental compositions, you can derive the empirical formula.
Now we need to find the smallest integer ratio. Carbon is already in integer
form but what factor multiplied by 2.25 gives an integer. The factor will be
4. This means we need to multiply each of these mole ratios by 4 to get the
C: 1 x 4 = 4
H: 2.25 x 4
So the empirical
formula is C4H9.
From the information given in this example, can we determine the molecular formula?
We know the molecular formula is a multiple of the empirical formula: (C4H9)x
The molecular mass will be the sum of the individual molecular masses
Therefore the molecular formula is (C4H9)2 = C8H18,
which is iso-octane.
help with a strategy?
Now you should have a better understanding of chemical formulas and the different
ways chemists represent a compound. Also, you should be able to determine percent
elemental compositions and know how to calculate empirical formulas from the
percent elemental composition.