As you have seen, a chemical formula is an indication of the makeup of a
compound in terms of the kinds of atoms and their relative numbers. It also has
some quantitative applications. By using the atomic masses assigned to the
elements, we can find the formula mass of a compound. If we are sure that the
formula represents the actual makeup of one molecule of the substance, the term
molecular mass may be used as well. In some cases the formula represents an
ionic lattice and no discrete molecule exists, as in the case of table salt, NaCl, or
the formula merely represents the simplest ratio of the combined substances and
not a molecule of the substance. For example, CH 2 is the simplest ratio of carbon
and hydrogen united to form the actual compound ethylene, C 2 H 4. This simplest
ratio formula is called the empirical formula, and the actual formula is the true
formula. The formula mass is determined by multiplying the atomic mass units (as
a whole number) by the subscript for that element and then adding these values for
all the elements in the formula. For example:
Ca(OH) 2 (one calcium amu + two hydrogen and two oxygen amu = formula
mass).
In Chapter 6, the concept of a mole is introduced. If you have 6.02 × 10^23
atoms of an element, then the atomic mass units can be expressed in grams, and
then the formula mass can be called the molar mass. Another example is Fe 2 O 3.
TIP
Know how to compute the percentage composition of an element in a compound.It is sometimes useful to know what percent of the total weight of a
compound is made up of a particular element. This is called finding the
percentage composition. The simple formula for this is:
To find the percent composition of calcium in calcium hydroxide in the