Chemistry Reference
In-Depth Information
EXAMPLE 5.1
How many atoms of each element are present in one formula unit of each of
the following compounds?
(a) Mg(ClO
3
)
2
(b)
(NH
4
)
2
SO
3
(c)
CH
3
CH
2
OH
(d)
S
8
Solution
(a) 1 Mg, 2 Cl, 6 O
(b) 2 N, 8 H, 1 S, 3 O
Figure 5.6
Blue Hydrate
CuSO
4
·5H
2
O (left), and White,
Anhydrous CuSO
4
(right)
(c) 2 C, 6 H, 1 O
(d) 8 S
Practice Problem 5.1
How many atoms of each element are present
in one formula unit of each of the following compounds?
(a) Hg
2
Cl
2
(b)
NH
4
H
2
PO
4
A few compounds have formulas that are written with a
centered dot,
as
in CuSO
4
·5H
2
O. In general, the dots connect the formulas of two or more com-
pounds that could exist independently but are bonded in some way in a single
compound. The coefficient (5) after the centered dot multiplies everything after
it until the end of the formula. Thus, 10 hydrogen atoms and 9 oxygen atoms
are in one formula unit of CuSO
4
·5H
2
O. This particular formula could have
been written CuSO
4
(H
2
O)
5
. When water is one of the compounds in a formula
with a centered dot, the combination is called a
hydrate.
The compound
with-
out
the attached water is said to be
anhydrous,
meaning “without water.” Anhy-
drous CuSO
4
and its hydrate are pictured in Figure 5.6.
EXAMPLE 5.2
What is the difference between the formulas (NH
4
)
3
PO
3
and N
3
H
12
PO
3
?
Solution
Although both formulas have the same ratios of atoms of each element to atoms
of all the others and to formula units of the compound, the first formula states
that the atoms of nitrogen and hydrogen are bonded together in some way in
each of three NH
4
groups.
Practice Problem 5.2
(a)
Write a formula that implies that a CH
3
group is connected to a CO
2
H
group by two CH
2
groups.
(b)
Write a formula showing just the numbers of atoms of each element in
this compound.
In reading formulas aloud, the number is simply stated for any subscript
following a symbol, as in H
2
O: “H two O.” To express parentheses followed
