Representation of Compounds
The formula for a chemical compound gives us information about the relative proportions of the
different elements that constitute it. Conversely, knowledge of the composition of a compound
enables us to determine its (empirical) formula. Knowing how to represent chemical compounds,
and knowing how to determine a compound’s formula, is very important for the SAT Subject Test:
Chemistry.
LAW OF CONSTANT COMPOSITION
The law of constant composition states that any sample of a given compound will contain the same
elements in the identical mass ratio. For instance, every sample of H 2 O will contain two atoms of
hydrogen for every atom of oxygen, or, in other words, one gram of hydrogen for every eight grams
of oxygen. This is hardly surprising since we already know that atoms prefer an octet structure and
would combine with other atoms in predictable ways to achieve this.
EMPIRICAL AND MOLECULAR FORMULAS
There are two ways to express a formula for a compound. The empirical formula gives the simplest
whole number ratio of the elements in the compound. The molecular formula gives the exact
number of atoms of each element in a molecule of the compound, and is a multiple of the empirical
formula (including a multiple of 1—that is, same as the empirical formula). For example, benzene is
a molecule where six carbon atoms are joined together in a ring, with a hydrogen atom attached to
each of them. Its molecular formula is therefore C 6 H 6 , but its empirical formula is just CH. For some
compounds, the empirical and molecular formulas are the same, as in the case of H 2 O. An ionic
compound, such as NaCl or CaCO 3 , will have only an empirical formula since there are no real
molecules in the solid state in these cases, as discussed above.