Solving Problems_ A Chemistry Handbook

CHAPTER 9 SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK

▲ Reactions that form water or a gas Some double-replacement
reactions in aqueous solution produce water or a gas (or both) rather
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. than a precipitate. In such cases, the water or gas is shown as a prod-
uct in the net ionic equation, as are the ions that produced it. The
remaining ions are eliminated as spectator ions. The following
example problem illustrates this concept.

Example Problem 9-5
Writing Equations for a Reaction That Produces Water

When hydrochloric acid and potassium hydroxide solutions are
mixed, water results, together with an aqueous solution of potassium
chloride. Write the balanced chemical equation, a complete ionic
equation, and a net ionic equation for this reaction.

The balanced chemical equation is the same as the skeleton equation.

HCl(aq) ϩ KOH(aq) 0 H2O(l) ϩ KCl(aq)

Write the complete ionic equation, which includes all of the ions.
Hϩ(aq) ϩ ClϪ(aq) ϩ Kϩ(aq) ϩ OHϪ(aq) 0

H2O(l) ϩ Kϩ(aq) ϩ ClϪ(aq)

Remove the spectator ions to produce the net ionic equation.
Hϩ(aq) ϩ OHϪ(aq) 0 H2O(l)

Practice Problems
7. Write balanced chemical, complete ionic, and net ionic equa-
tions for the reactions between the following substances, which
produce water.
a. nitric acid (HNO3) and aqueous barium hydroxide
b. sulfuric acid (H2SO4) and aqueous sodium hydroxide
c. phosphoric acid (H3PO4) and aqueous lithium hydroxide

Example Problem 9-6
Writing Equations for a Reaction That Produces a Gas

Write balanced chemical, complete ionic, and net ionic equations for
the reaction between aqueous solutions of sodium sulfide and
hydrochloric acid. Hydrogen sulfide gas is produced, along with an
aqueous solution of sodium chloride.

Solving Problems: A Chemistry Handbook Chemistry: Matter and Change 89

CHAPTER 9 SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK

Write the skeleton equation with the correct formulas. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Na2S(aq) ϩ HCl(aq) 0 NaCl(aq) ϩ H2S(g)
Add coefficients to write a balanced chemical equation.

Na2S(aq) ϩ 2HCl(aq) 0 2NaCl(aq) ϩ H2S(g)
Write the complete ionic equation, which includes all the ions.
2Naϩ(aq) ϩ S2Ϫ(aq) ϩ 2Hϩ(aq) ϩ 2ClϪ(aq) 0

2Naϩ(aq) ϩ 2ClϪ(aq) ϩ H2S(g)
Remove the spectator ions to produce the net ionic equation.

S2Ϫ(aq) ϩ 2Hϩ(aq) 0 H2S(g)

Practice Problems
8. Write balanced chemical, complete ionic, and net ionic equa-
tions for the reactions between the following substances, which
produce a gas.
a. hydrochloric acid and aqueous sodium cyanide, with produc-
tion of hydrogen cyanide gas (HCN)
b. sulfuric acid (H2SO4) and aqueous rubidium sulfide, with
production of hydrogen sulfide gas

Chapter 9 Review
9. Define chemical reaction. Where are the reactants and products
shown in an equation for a chemical reaction, and how are the
physical states indicated?

10. Compare and contrast word equations, skeleton equations, and
balanced chemical equations.

11. Briefly list the steps used in balancing a chemical equation.
12. Contrast synthesis, combustion, and decomposition reactions.
13. Contrast single-replacement and double-replacement reactions.
14. Define aqueous solution. What name is given to a solid that

forms when two aqueous solutions are mixed?
15. How do complete ionic and net ionic equations differ?
16. Name three typical types of products that may be formed in

double-replacement reactions.

90 Chemistry: Matter and Change Solving Problems: A Chemistry Handbook

CHAPTER 10 SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK

The Mole

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. 10.1 Measuring Matter

Substances react according to definite ratios of numbers of particles
(atoms, ions, formula units, or molecules). The following balanced
chemical equation shows that two atoms of aluminum react with
three molecules of iodine to form two formula units of aluminum
iodide.

2Al(s) ϩ 3I2(s) 0 2AlI3(s)

If you wanted to carry out this reaction, how could you measure the
correct numbers of particles of aluminum and iodine? Fortunately,
you can count particles by measuring mass. Suppose you have a
sample of an element, and the mass of the sample in grams is
numerically equal to the atomic mass of the element. Scientists have
discovered that this mass of an element contains 6.02 ϫ 1023 atoms
of that element. This number is called Avogadro’s number.
Avogadro’s number of particles is called a mole of particles. The
mole is the SI base unit used to measure the amount of a substance
and is defined as the number of particles in exactly 12 g of pure
carbon-12. For the purpose of dealing with moles, the simplest unit
of any substance is called a representative particle. The particle may
be an atom, a molecule, a formula unit, or an ion.

Example Problem 10-1
Converting Moles to Number of Particles

How many molecules are in 2.25 moles of bromine (Br2)?
Bromine is an element that consists of diatomic molecules.
Therefore, one mole of bromine contains 6.02 ϫ 1023 Br2 molecules.
To find the number of Br2 molecules present in 2.25 mol, multiply
the number of moles by Avogadro’s number.

number of Br2 molecules ϭ
2.25 mol ϫ ᎏ6.02 ϫ 1ᎏ0213 mmoollecᎏules Br2

number of Br2 molecules ϭ 1.35 ϫ 1024 molecules Br2

Solving Problems: A Chemistry Handbook Chemistry: Matter and Change 91

CHAPTER 10 SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK

Example Problem 10-2
Converting Number of Particles to Moles

Calculate the number of moles in a sample of sodium bromide
(NaBr) that contains 2.88 ϫ 1023 formula units.
Because 1 mol NaBr ϭ 6.02 ϫ 1023 formula units NaBr, you can
see that 2.88 ϫ 1023 formula units is less than one mole of NaBr.

moles of NaBr ϭ
2.88 ϫ 1023 formula units ϫ ᎏ6.02 ϫ11ᎏ0m2o3lfoNramBᎏurla units

moles of NaBr ϭ 0.478 mol NaBr

Practice Problems Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

1. Calculate the number of molecules in 15.7 mol carbon dioxide.

2. Calculate the number of molecules in 0.0544 mol H2O.
3. Calculate the number of moles in 9.22 ϫ 1023 atom iron.

4. Calculate the number of moles of sucrose in a sample that con-
tains 2.05 ϫ 1022 sucrose molecules.

5. A student uses 0.0850 mol copper sulfate to carry out a reac-
tion. If the reaction uses up 0.0832 mol copper sulfate, how
many formula units are left unreacted?

10.2 Mass and the Mole

One mole of a monatomic element consists of 6.02 ϫ 1023 atoms of
that element. The mass of a mole of any substance is called the
molar mass of the substance. For example, the molar mass of a
monatomic element is numerically equal to the atomic mass of the
element, but expressed in grams.

molar mass of a substance ϭ ᎏg1rmamosl ooᎏff tthhee ssuubᎏbssttaannccee

This relationship can be used to convert between mass and moles.

92 Chemistry: Matter and Change Solving Problems: A Chemistry Handbook