Chemistry Reference
In-Depth Information
The meaning of a chemical formula was discussed in Chapter 5, and we learned
how to interpret formulas in terms of the numbers of atoms of each element per
formula unit. In this chapter, we will learn how to calculate the number of grams
of each element in any given quantity of a compound from its formula and to
do other calculations involving formulas. Formula masses are presented in
Section 7.1, and percent composition is considered in Section 7.2. Section 7.3
discusses the mole—the basic chemical quantity of any substance. Moles can
be used to count atoms, molecules, or ions and to calculate the mass of any
known number of formula units of a substance. Section 7.4 shows how to use
relative mass data to determine empirical formulas, and the method is extended
to molecular formulas in Section 7.5.
7.1
Formula Masses
Because each symbol in a formula represents an atom, which has a given aver-
age atomic mass, the formula as a whole represents a collection of atoms with
a given formula mass. The
formula mass
(also called
formula weight
) is the
sum of the atomic masses of all atoms of every element (not merely each type
of atom) in a formula unit. In general, formula masses should be calculated to
as many significant digits as are given in any data presented in a problem. For
problems in which no data are given, at least three significant digits should be
used in values from the periodic table. For example, we can calculate the for-
mula mass for acetic acid, the major acid in vinegar,
HC
2
H
3
O
2
,
as follows:
Number of
atoms per
Atomic
formula unit
mass
4 H
2 C
2 O
4
1.01 amu
4.04 amu
2
12.0 amu
24.0 amu
2
16.0 amu
32.0 amu
Formula mass
60.0 amu
Because calculation of formula mass is essentially adding two or more num-
bers, the numbers to be added may be rounded to the same number of decimal
places. For example, the four H atoms in the preceding calculation might have
been included as 4.0 amu. Prior multiplication might affect the number of dec-
imal places we retain in the atomic masses (see Problem 7.14).
The three types of formula masses correspond to the three types of formula
units: (1) atomic masses (also called atomic weights), (2)
molecular masses
(also called
molecular weights
), and (3) formula masses for ionic compounds
(also called formula weights). The term “atomic mass” may be used whether an
atom is combined or not, but it always refers to the mass of one atom of an
element.
EXAMPLE 7.1
(a) What is the atomic mass of nitrogen?
(b) What is the molecular mass of nitrogen?
