Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
68 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
7
Example Problem 7-4
The Formula for a Compound Containing a Polyatomic Ion
Write the chemical formula for a compound formed from ammonium ions and sulfate ions. Refer to a table of common ions as
necessary.
The ammonium ion has the formula NH 4
ϩ . The sulfate ion has the
formula SO 4
2Ϫ . The overall charge of the formula unit must be zero,
which means that there must be two ammonium ions for each sulfate
ion. Thus, the correct formula is (NH 4 ) 2 SO 4 . The subscript after the
parenthesis indicates the presence of two ammonium ions.
Practice Problems
Write the correct formula for the ionic compounds that
contain the following ions. Refer to a table of polyatomic
ions as necessary.
16. sodium and phosphate
17. ammonium and carbonate
18. aluminum and chromate
19. calcium and hydroxide
Naming ionic compounds In naming ionic compounds, name
the cation first, then the anion. Monatomic cations use the element
name. Monatomic anions use the root of the element name plus the
suffix -ide. If an element can have more than one oxidation number,
use a Roman numeral in parentheses after the element name, for
example, iron(II) to indicate the Fe 2ϩ ion. For polyatomic ions, use
the name of the ion.
Certain polyatomic ions, called oxyanions, contain oxygen and
another element. If two different oxyanions can be formed by an
element, the suffix -ate is used for the oxyanion containing more
oxygen atoms, and the suffix -ite for the oxyanion containing fewer
oxygens. In the case of the oxyanions of the halogens, the following
special rules are used.
four oxygens, per ϩ root ϩ -ate (example: perchlorate, ClO 4
Ϫ )
three oxygens, root ϩ -ate (example: chlorate, ClO 3
Ϫ )
two oxygens, root ϩ -ite (example: chlorite, ClO 2
Ϫ )
one oxygen, hypo- ϩ root ϩ -ite (example: hypochlorite, ClO Ϫ )
▲
68 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER
7
Example Problem 7-4
The Formula for a Compound Containing a Polyatomic Ion
Write the chemical formula for a compound formed from ammonium ions and sulfate ions. Refer to a table of common ions as
necessary.
The ammonium ion has the formula NH 4
ϩ . The sulfate ion has the
formula SO 4
2Ϫ . The overall charge of the formula unit must be zero,
which means that there must be two ammonium ions for each sulfate
ion. Thus, the correct formula is (NH 4 ) 2 SO 4 . The subscript after the
parenthesis indicates the presence of two ammonium ions.
Practice Problems
Write the correct formula for the ionic compounds that
contain the following ions. Refer to a table of polyatomic
ions as necessary.
16. sodium and phosphate
17. ammonium and carbonate
18. aluminum and chromate
19. calcium and hydroxide
Naming ionic compounds In naming ionic compounds, name
the cation first, then the anion. Monatomic cations use the element
name. Monatomic anions use the root of the element name plus the
suffix -ide. If an element can have more than one oxidation number,
use a Roman numeral in parentheses after the element name, for
example, iron(II) to indicate the Fe 2ϩ ion. For polyatomic ions, use
the name of the ion.
Certain polyatomic ions, called oxyanions, contain oxygen and
another element. If two different oxyanions can be formed by an
element, the suffix -ate is used for the oxyanion containing more
oxygen atoms, and the suffix -ite for the oxyanion containing fewer
oxygens. In the case of the oxyanions of the halogens, the following
special rules are used.
four oxygens, per ϩ root ϩ -ate (example: perchlorate, ClO 4
Ϫ )
three oxygens, root ϩ -ate (example: chlorate, ClO 3
Ϫ )
two oxygens, root ϩ -ite (example: chlorite, ClO 2
Ϫ )
one oxygen, hypo- ϩ root ϩ -ite (example: hypochlorite, ClO Ϫ )
▲