Chemistry Reference
In-Depth Information
Lesson 5-4: Molecular and Empirical Formulas
At this point, we should distinguish between two important types of
formulas that you will encounter during your study of chemistry. Each
molecular compound will have its own unique molecular formula, as well
as an empirical formula, which it may share with one or more other com-
pounds. The molecular formula of a compound represents the particles that
make up the compound, as they actually exist. For example, glucose, with
the molecular formula C
6
H
12
O
6
, actually exists in 24-atom molecules, as
the molecular formula suggests. The empirical formula of a compound shows
the simplest whole number ratio for the elements that make up the com-
pound. So, for example, if we reduced all of the subscript numbers in the
molecular formula for glucose by dividing each by the number 6, we would
get the empirical formula for glucose: CH
2
O.
Notice that the empirical formula for glucose gives less information
than the molecular formula, simply noting the fact that there are twice as
many hydrogen atoms as either carbon or oxygen atoms. You may wonder
why anyone would be interested in the empirical formula if it contains less
information. The answer to that is very simple: It is easier to experimen-
tally determine the empirical formula of a compound than the molecular
formula of a compound, so that the empirical formula is often found first,
as a step towards finding the molecular formula.
Comparing Molecular and Empirical Formulas
Molecular
Formula
Empirical
Formula
Molecular
Formula
Empirical
Formula
H
2
O
2
N
2
O
4
CH
2
HO
H
2
O
C
8
H
16
C
2
H
4
H
2
O
CH
2
CH
2
NO
2
CH
2
Figure 5-4a
You should notice two interesting things from Figure 5-4a. First, no-
tice that some molecular formulas can't be reduced, as in the case of H
2
O,
so sometimes the molecular and empirical formulas for a compound are
identical. Next, notice that some molecular compounds, such as C
2
H
4
and
C
8
H
16
, have exactly the same empirical formula. As I mentioned previously,
each compound has its own unique molecular formula, but several com-
pounds can share the same empirical formula.
