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 formula. 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 empirical formula.
C: 1 x 4 = 4
H: 2.25 x 4 = 9
So the empirical formula is C_{4}H_{9}.
From the information given in this example, can we determine the molecular formula? Answer
We know the molecular formula is a multiple of the empirical formula: (C_{4}H_{9})_{x}
The molecular mass will be the sum of the individual molecular masses
_{}
Therefore the molecular formula is (C_{4}H_{9})_{2} = C_{8}H_{18}, which is iso-octane.
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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.