Page 114 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 7] FORMULA CALCULATIONS 103

The word molecule or the general term formula unit may be applied to one unit of H 2 O. The word atom
or the term formula unit may be applied to one unit of uncombined Pt. However, there is no special name for
one unit of NaCl. Formula unit is the best designation. (Some instructors and some texts refer to “molecules” of
NaCl, and especially to “molecular mass” of NaCl, because the calculations done on formula units do not depend
on the type of bonding involved. However, strictly speaking, the terms molecule and molecular mass should be
reserved for substances bonded into molecules.)

7.3. FORMULA MASSES
The formula mass of a compound is the sum of the atomic masses (Chap. 3) of all the atoms (not merely
each kind of atom) in the formula. Thus, in the same way that a symbol is used to represent an element, a
formula is used to represent a compound or a molecule of an element, such as H 2 , and also one unit of either.
The formula mass of the substance or the mass of 1 mol of the substance is easily determined on the basis of the
formula (Sec. 7.4). Note that just as formula unit may refer to uncombined atoms, molecules, or atoms combined
in an ionic compound, the term formula mass may refer to the atomic mass of an atom, the molecular mass of a
molecule, or the formula mass of a formula unit of an ionic compound.
Use at least three significant digits in formula mass calculations.

EXAMPLE 7.2. What is the formula mass of CaSO 4 ?
Ans. Atomic mass of calcium = 40.08 amu
Atomic mass of sulfur = 32.06 amu
4 × Atomic mass of oxygen = 4 × 16.00 amu = 64.00 amu
Formula mass = total = 136.14 amu

7.4. THE MOLE
Atoms and molecules are incredibly small. For hundreds of years after Dalton postulated their existence,
no one was able to work with just one atom or molecule. (In recent times, with special apparatus, it has been
possible to see the effects of individual atoms and molecules, but this subject will be developed later.) Just as the
dozen is used as a convenient number of items in everyday life, the mole may be best thought of as as number
of items. The mole is 6.02 × 10 23 items, a number called Avogadro’s number. This is a very large number:
six hundred two thousand billion billion! The entire earth has a mass of 6 × 10 24 kg. Thus, the earth has only
10 times as many kilograms as 1 mol of carbon has atoms. One can have a mole of any item, but it makes little
sense to speak of moles of anything but the tiniest of particles, such as atoms and molecules. It might seem
unusual to give a name to a number, but remember we do the same thing in everyday life; dozen is the name
for 12 items. Just as a grocer finds selling eggs by the dozen more convenient than selling them individually,
the chemist finds calculations more convenient with moles. The number of formula units (i.e., the number of
uncombined atoms, of molecules of molecular elements or compounds, or of formula units of ionic compounds)
can be converted to moles of the same substance, and vice versa, using Avogadro’s number (Fig. 7-1).

Number of Avogadro’s number Number of
formula units moles
Fig. 7-1. Avogadro’s number conversions

Mole is abbreviated mol. Do not use m or M for mole; these symbols are used for other quantities related
to moles, and so you will be confused if you use either of them. Note: A mole is referred to by some authors as
a “gram molecular mass” because 1 mol of molecules has a mass in grams equal to its molecular mass. In this
terminology, a “gram atomic mass” is 1 mol of atoms, and a “gram formula mass” is 1 mol of formula units.
The formula mass of a substance is equal to its number of grams per mole. Avogadro’s number is the number
of atomic mass units in 1 g. It is defined in that manner so that the atomic mass of an element (in amu) is