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that the approximate (rounded) atomic weight of hydrogen is 1.0 u
and oxygen is 16.0 u. Adding the atomic weights for all the atoms
gives the following:
Atoms Atomic Weight Totals
2 of H 2 × 1.0 u = 2.0 u
1 of O 1 × 16.0 u = 16.0 u
Formula weight = 18.0 u
Thus, the formula weight of a water molecule is 18.0 u.
The formula weight of an ionic compound is found in the
same way, by adding the rounded atomic weights of atoms (or
ions) making up the compound. Sodium chloride is NaCl, so
the formula weight is 23.0 u plus 35.5 u, or 58.5 u. The formula
weight can be calculated for an ionic or molecular substance. The
molecular weight is the formula weight of a molecular substance.
The term molecular weight is sometimes used for all substances,
whether or not they have molecules. Since ionic substances such
as NaCl do not occur as molecules, this is not strictly correct.
Both molecular and formula weights are calculated in the same
way, but formula weight is a more general term.
FIGURE 10.1 The products of chemical research have sub-
stantially increased food supplies but have also increased the pos-
sibilities of pollution. Balancing the benefits and hazards of the use EXAMPLE 10.1
of chemicals requires a knowledge of chemistry and a knowledge of What is the molecular weight of table sugar (sucrose), which has the
the alternatives. formula C 12 H 22 O 11 ?

SOLUTION
Molecular Structural
Name formula Space-filled formula The formula identifies the numbers of each atom, and the atomic
weights are from a periodic table:
O
Water H 2 O O H H Atoms Atomic Weight Totals
H H =
12 of C 12 × 12.0 u 144.0 u
22 of H 22 × 1.0 u = 22.0 u
N 11 of O 11 × 16.0 u = 176.0 u
Ammonia NH 3 H N H Molecular weight = 342.0 u
H H H H

EXAMPLE 10.2
H
H
Hydrogen O O What is the molecular weight of ethyl alcohol, C 2 H 5 OH? (Answer: 46.0 u)
peroxide H 2 O 2 H O O
H

Carbon PERCENT COMPOSITION OF COMPOUNDS
dioxide CO 2 O C O O C O
The formula weight of a compound can provide useful informa-
tion about the elements making up a compound (Figure 10.3).
For example, suppose you want to know how much calcium is
FIGURE 10.2 The name, molecular formula, sketch, and
structural formula of some common molecules. Compare the kinds provided by a dietary supplement. The label lists the ingredient
and numbers of atoms making up each molecule in the sketch to as calcium carbonate, CaCO 3 . To find out how much calcium is
the molecular formula. supplied by a pill with a certain mass, you need to find the mass
percentage of calcium in the compound.
The mass percentage of an element in a compound can be
MOLECULAR AND FORMULA WEIGHTS found from
The formula weight of a compound is the sum of the atomic
)
atomic weight
(
number of atoms
weights of all the atoms in a chemical formula. For example, the for- ( )

of element
of element
% of
100% of
×

mula for water is H 2 O. Hydrogen and oxygen are both non metals, ____ =

compound
so the formula means that one atom of oxygen is bound to two formula weight of compound element
hydrogen atoms in a molecule. From the periodic table, you know equation 10.1
10-3 CHAPTER 10 Chemical Reactions 253