9
Chemical 117
Chapter Calculations and Chemical Formulas
Review Skills
9.1 A Typical Problem
9.2 Relating Mass to Number of Particles
Atomic Mass and Counting Atoms by Weighing
Molar Mass
9.3 Molar Mass and Chemical Compounds
Molecular Mass and Molar Mass of Molecular Compounds
Ionic Compounds, Formula Units, and Formula Mass
Internet: Molar Mass Conversion Factors
9.4 Relationships Between Masses of Elements and Compounds
Internet: Percentage of an Element in a Compound
9.5 Determination of Empirical and Molecular Formulas
Determining Empirical Formulas
Converting Empirical Formulas Into Molecular Formulas
Special Topic 9.1: Green Chemistry - Making Chemicals from Safer Reactants
Internet: Combustion Analysis
Special Topic 9.2: Safe and Effective?
Chapter Glossary
Internet: Glossary Quiz
Chapter Objectives
Review Questions
Key Ideas
Chapter Problems
Section Goals and Introductions
Section 9.1 A Typical Problem
Goal: To introduce the chapter by describing a typical problem that you will be able to work
after studying the chapter.
Sometimes a task becomes much easier if you know from the beginning where you are going
to end up. It will help you to understand the importance of Sections 9.2 and 9.3 if you first
know how the calculations described there can be used. This section shows a typical problem
and gives you a sense of why it is important.
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Study Guide for An Introduction to Chemistry
Section 9.2 Relating Mass to Number of Particles
Goals
To show how to do a procedure called counting by weighing.
To introduce atomic mass and show how it can be used to convert between the mass of a
sample of an element and the number of atoms that the sample contains.
Even a tiny sample of an element contains a huge number of atoms. Theres no way that you
could ever count that high, even if you were able to count atoms one at a time (which you
cant). So if you want to know the number of atoms in a sample of an element, you have to do
it by an indirect technique called counting by weighing. This section introduces this technique
and shows how it can be applied to the conversions between mass of a sample of an element
and the number of atoms in the sample. An important unit called the mole is introduced in this
section. It is very important that you understand what it is and how it is used.
Section 9.3 Molar Mass and Chemical Compounds
Goal: To introduce molecular mass and formula mass and show how they can be used to
convert between the mass of a sample of a compound and the number of molecules or formula
units that the sample contains.
This section shows how to calculate the number of molecules (expressed in moles) in a
sample of a molecular compound from the mass of that sample and how to calculate the mass
in a sample of a molecular compound from the moles of molecules it contains. The section
also explains why ionic compounds do not contain molecules and how the term formula unit
can be used to describe the units of ionic compounds that are like molecules of molecular
compounds. Then you will see how to calculate the number of formula units in a sample of an
ionic compound (expressed in moles) from the mass of that sample and how to calculate the
mass of an ionic compound and the moles of formula units it contains. These calculations are
very commonly done by chemists and chemistry students, so be sure you can do them quickly
and correctly.
Internet: Molar Mass Conversion Factors
Section 9.4 Relationships Between Masses of Elements and Compounds
Goal: To show how you convert between mass of an element and mass of a compound that
contains the element.
This section shows how you can use the skills you learned in Section 9.2 and 9.3 and some
information derived from chemical formulas for compounds to convert between mass of an
element and mass of a compound that contains the element.
The section on our Web site called Percentage of an Element in a Compound describes
calculations that are related to this section.
Internet: Percentage of an Element in a Compound
Section 9.5 Determination of Empirical and Molecular Formulas
Goal: To describe what empirical and molecular formulas are and how they can be
determined.
All compounds can be described with empirical formulas, and molecular compounds can also
be described with molecular formulas. This section describes the information given by each
type of formula and shows ways to determine them. A section on our Web site describes an
experimental technique used to determine empirical formulas.
Internet: Combustion Analysis
Chapter 9 Chemical Calculations and Chemical Formulas
119
Chapter 9 Map
Chapter Checklist
Read the Review Skills section. If there is any skill mentioned that you have not yet
mastered, review the material on that topic before reading this chapter.
Read the chapter quickly before the lecture that describes it.
Attend class meetings, take notes, and participate in class discussions.
Work the Chapter Exercises, perhaps using the Chapter Examples as guides.
Study the Chapter Glossary and test yourself on our Web site:
Internet: Glossary Quiz
Study all of the Chapter Objectives. You might want to write a description of how you
will meet each objective. (Although it is best to master all of the objectives, the following
objectives are especially important because they pertain to skills that you will need while
studying other chapters of this text: 4, 5, 7, 10, and 12.)
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Study Guide for An Introduction to Chemistry
Reread the Study Sheets in this chapter and decide whether you will use them or some
variation on them to complete the tasks they describe.
Sample Study Sheet 9.1: Converting Between Mass of Element and Mass of
Compound Containing the Element
Sample Study Sheet 9.2: Calculating Empirical Formulas
Sample Study Sheet 9.3: Calculating Molecular Formulas
To get a review of the most important topics in the chapter, fill in the blanks in the Key
Ideas section.
Work all of the selected problems at the end of the chapter, and check your answers with
the solutions provided in this chapter of the study guide.
Ask for help if you need it.
Web Resources
Internet: Molar Mass Conversion Factors
Internet: Percentage of an Element in a Compound
Internet: Combustion Analysis
Internet: Glossary Quiz
Exercises Key
Exercise 9.1 - Atomic Mass Calculations: Gold is often sold in units of troy ounces.
There are 31.10 grams per troy ounce. (Objs 3 & 4)
a. What is the atomic mass of gold?
196.9665 (from periodic table)
b. What is the mass in grams of 6.022 1023 gold atoms?
196.9665 g (There are 6.022 1023 atoms per mole of atoms, and one mole of an
element has a mass in grams equal to its atomic mass.)
c. Write the molar mass of gold as a conversion factor that can be used to convert
between grams of gold and moles of gold.
196.9665 g Au
1 mol Au
d. What is the mass in grams of 0.20443 mole of gold?
196.9665 g Au
? g Au = 0.20443 mol Au
= 40.266 g Au
1 mol Au
e. What is the mass in milligrams of 7.046 103 mole of gold?
3
196.9665 g Au 10 mg
? mg Au = 7.046 103 mol Au
= 1388 mg Au
1 mol Au 1 g
f. How many moles of gold are in 1.00 troy ounce of pure gold?
31.10 g 1 mol Au
? mol Au = 1.00 troy oz Au
= 0.158 mol Au
1 troy oz 196.9665 g Au
Chapter 9 Chemical Calculations and Chemical Formulas
121
Exercise 9.2 - Molecular Mass Calculations: A typical glass of wine contains about
16 g of ethanol, C2H5OH. (Objs 5-7)
a. What is the molecular mass of C2H5OH?
2(12.011) + 6(1.00794) + 1(15.9994) = 46.069
b. What is the mass of 1 mole of C2H5OH?
46.069 g (One mole of a molecular compound has a mass in grams equal to its
molecular mass.)
c. Write a conversion factor that will convert between mass and moles of C2H5OH.
46.069 g C2 H 5OH
1 mol C2 H 5OH
d. How many moles of ethanol are there in 16 grams of C2H5OH?
1 mol C2 H 5OH
? mol C2 H 5OH = 16 g C2 H 5OH
= 0.35 mole C2H5OH
46.069 g C 2 H 5OH
e. What is the volume in milliliters of 1.0 mole of pure C2H5OH? (The density of
ethanol is 0.7893 g/mL.)
46.069 g C 2 H 5OH 1 mL C 2 H 5OH
? mL C 2 H 5OH = 1.0 mol C2 H 5OH
1 mol C2 H 5OH 0.7893 g C 2 H 5OH
= 58 mL C2H5OH
Exercise 9.3 - Formula Mass Calculations: A quarter teaspoon of a typical baking
powder contains about 0.4 g of sodium hydrogen carbonate, NaHCO3.
a. Calculate the formula mass of sodium hydrogen carbonate.
Formula Mass = 1(22.9898) + 1(1.00794) + 1(12.011) + 3(15.9994)
= 84.007
b. What is the mass in grams of 1 mole of NaHCO3?
84.007 g (One mole of an ionic compound has a mass in grams equal to its
formula mass.)
c. Write a conversion factor to convert between mass and moles of NaHCO3.
84.007 g NaHCO 3
1 mol NaHCO 3
d. How many moles of NaHCO3 are in 0.4 g of NaHCO3?
1 mol NaHCO3
3
? mol NaHCO3 = 0.4 g NaHCO3
= 5 10 mol NaHCO3
84.007 g NaHCO3
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Study Guide for An Introduction to Chemistry
Exercise 9.4 - Molar Ratios of Element to Compound: Find the requested conversion
factors. (Objs 13 & 14)
a. Write a conversion factor that converts between moles of hydrogen and moles of
C2H5OH.
6 mol H
1 mol C2 H 5OH
b. Write a conversion factor that converts between moles of oxygen and moles of
NaHCO3.
3 mol O
1 mol NaHCO 3
c. How many moles of hydrogen carbonate ions, HCO3, are there in 1 mole of
NaHCO3?
There is 1 mole of HCO3 per 1 mole of NaHCO3.
Exercise 9.5 - Molar Mass Calculations: Disulfur dichloride, S2Cl2, is used in
vulcanizing rubber and in hardening soft woods. It can be made from the reaction of pure sulfur
with chlorine gas. What is the mass of S2Cl2 that contains 123.8 g S?
1 mol S 1 mol S2 Cl2 135.037 g S2 Cl2
? g S2 Cl2 = 123.8 g S
= 260.7 g S2Cl2
32.066 g S 2 mol S 1 mole S2 Cl2
Exercise 9.6 - Molar Mass Calculations: Vanadium metal, which is used as a component
of steel and to catalyze various industrial reactions, is produced from the reaction of
vanadium(V) oxide, V2O5, and calcium metal. What is the mass (in kilograms of vanadium) in
2.3 kilograms of V2O5? (Obj 15)
103 g 1 mol V2 O5 2 mol V 50.9415 g V 1 kg
? kg V = 2.3 kg V2 O5
3
1 kg 181.880 g V2 O5 1 mol V2 O5 1 mol V 10 g
= 1.3 kg V
Exercise 9.7 Calculating an Empirical Formula: Bismuth ore, often called bismuth
glance, contains an ionic compound that consists of the elements bismuth and sulfur. A sample
of the pure compound is found to contain 32.516 g Bi and 7.484 g S. What is the empirical
formula for this compound? What is its name? (Obj 16)
1 mol Bi
? mol Bi = 32.516 g Bi
= 0.15559 mol Bi 0.15559 = 1 mol Bi 2 = 2 mol Bi
208.9804 g Bi
1 mol S
1
? mol S = 7.484 g S
= 0.2334 mol S 0.15559 1 mol S 2 = 3 mol S
2
32.066 g S
Our empirical formula is Bi2S3 or bismuth(III) sulfide.
Chapter 9 Chemical Calculations and Chemical Formulas
123
Exercise 9.8 - Calculating an Empirical Formula: An ionic compound used in the
brewing industry to clean casks and vats and in the wine industry to kill undesirable yeasts and
bacteria is composed of 35.172% potassium, 28.846% sulfur, and 35.982% oxygen. What is the
empirical formula for this compound? (Obj 17)
1 mol K
? mol K = 35.172 g K
= 0.89958 mol K 0.89958 = 1 mol K 2 = 2 mol K
39.0983 g K
1 mol S
? mol S = 28.846 g S
= 0.89958 mol S 0.89958 = 1 mol S 2 = 2 mol S
32.066 g S
1 mol O
1
? mol O = 35.982 g O
= 2.2490 mol O 0.89958 2 mol O 2 = 5 mol O
2
15.9994 g O
Empirical Formula K2S2O5
Exercise 9.9 Calculating a Molecular Formula Using the Percentage of Each
Element in a Compound: Compounds called polychlorinated biphenyls (PCBs) have
structures similar to chlorinated insecticides such as DDT. They have been used in the past for a
variety of purposes, but because they have been identified as serious pollutants, their only legal
use today is as insulating fluids in electrical transformers. This is a use for which no suitable
substitute has been found. One PCB is 39.94% carbon, 1.12% hydrogen, and 58.94% chlorine
and has a molecular mass of 360.88. What is its molecular formula? (Obj 19)
1 mol C
? mol C = 39.94 g C
= 3.325 mol C 1.11 = 3 mol C 2 = 6 mol C
12.011 g C
1 mol H
? mol H = 1.12 g H
= 1.11 mol H 1.11 = 1 mol H 2 = 2 mol H
1.00794 g H
1 mol Cl
1
? mol Cl = 58.94 g Cl
= 1.662 mol Cl 1.11 1 mol Cl 2 = 3 mol Cl
2
35.4527 g Cl
molecular mass
360.88
n=
=
2
Empirical Formula C6H2Cl3
empirical formula mass 180.440
Molecular Formula = (C6H2Cl3)2 or C12H4Cl6
Review Questions Key
1. Complete each of the following conversion factors by filling in the blank on the top of the
ratio.
103 g
103 kg
a.
c.
1 kg
1 metric ton
103 mg
b.
1g
106 g
d.
1g
2. Convert 3.45 104 kg into grams.
103 g
7
? g = 3.45 104 kg
= 3.45 10 g
1 kg
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Study Guide for An Introduction to Chemistry
3. Convert 184.570 g into kilograms.
1 kg
? kg = 184.570 g 3 = 0.184570 kg
10 g
4. Convert 4.5000 106 g into megagrams.
1 Mg
? Mg = 4.5000 106 g 6 = 4.5000 Mg
10 g
5. Convert 871 Mg into grams.
106 g
8
? g = 871 Mg
= 8.71 10 g
1 Mg
6. Surinam bauxite is an ore that is 54-57% aluminum oxide, Al2O3. What is the mass (in
kilograms) of Al2O3 in 1256 kg of Surinam bauxite that is 55.3% Al2O3?
55.3 kg Al2 O3
? kg Al2 O3 = 1256 kg Surinam bauxite
= 695 kg Al2O3
100 kg Surinam bauxite
Key Ideas Answers
7. Because of the size and number of carbon atoms in any normal sample of carbon, it is
impossible to count the atoms directly.
9. The atomic mass of any element is the weighted average of the masses of the naturally
occurring isotopes of the element.
11. The number of grams in the molar mass of an element is the same as the elements atomic
mass.
13. In this text, the term formula unit is used to describe ionic compounds in situations where
molecule is used to describe molecular substances. It is the group represented by the
substances chemical formula, that is, a group containing the kinds and numbers of atoms or
ions listed in the chemical formula.
15. Formula mass is the weighted average of the masses of the naturally occurring formula units
of the substance.
17. When the subscripts in a chemical formula represent the simplest ratio of the kinds of atoms
in the compound, the formula is called an empirical formula.
19. The subscripts in a molecular formula are always whole-number multiples of the subscripts
in the empirical formula.
Chapter 9 Chemical Calculations and Chemical Formulas
125
Problems Key
Section 9.2 Relating Mass to Numbers of Particles
21. What is the weighted average mass in atomic mass units (u) of each atom of the following
elements?
The atomic mass for each element in the periodic table tells you the weighted average
mass in atomic mass units (u) of each atom of that element.
a. sodium 22.9898 u
b. oxygen 15.9994 u
23. What is the weighted average mass in grams of 6.022 1023 atoms of the following
elements?
The atomic mass for each element in the periodic table tells you the mass, in grams, of
6.022 1023 atoms of that element.
a. sulfur 32.066 g
b. fluorine 18.9984 g
25. What is the molar mass for each of the following elements?
The atomic mass for each element in the periodic table tells you the molar mass, in grams
per mole, of that element.
a. zinc 65.39 g/mol
b. aluminum 26.9815 g/mol
27. For each of the following elements, write a conversion factor that converts between mass in
grams and moles of the substance. (Obj 3)
55.845 g Fe
1 mol Fe
83.80 g Kr
b. krypton
1 mol Kr
29. A vitamin supplement contains 50 micrograms of the element selenium in each tablet. How
many moles of selenium does each tablet contain? (Obj 4)
a. iron
1 g 1 mol Se
7
? mol Se = 50 g Se 6
= 6.3 10 mol Se
10 g 78.96 g Se
31. A multivitamin tablet contains 1.6 104 mole of iron per tablet. How many milligrams of
iron does each tablet contain? (Obj 4)
3
55.845 g Fe 10 mg
? mg Fe = 1.6 10 mol Fe
= 8.9 mg Fe
1 mol Fe 1 g
4
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Study Guide for An Introduction to Chemistry
Section 9.3 Molar Mass and Chemical Compounds
33. For each of the following molecular substances, calculate its molecular mass and write a
conversion factor that converts between mass in grams and moles of the substance.
(Objs 5 & 6)
a. H3PO2
molecular mass H3PO2 = 3(1.00794) + 1(30.9738) + 2(15.9994)
65.9964 g H 3 PO 2
= 65.9964 leads to
1 mol H 3 PO 2
b. C6H5NH2
molecular mass C6H5NH2 = 6(12.011) + 7(1.00794) + 1(14.0067)
93.128 g C6 H 5 NH 2
= 93.128 leads to
1 mol C6 H 5 NH 2
35. Each dose of nighttime cold medicine contains 1000 mg of the analgesic acetaminophen.
Acetaminophen, or N-acetyl-p-aminophenol, has the general formula C8H9NO. (Obj 7)
a. How many moles of acetaminophen are in each dose?
1 g 1 mol C8 H 9 NO
? mol C8 H 9 NO = 1000 mg C8 H 9 NO 3
10 mg 135.166 g C8 H 9 NO
= 7.398 103 mol C8H9NO
b. What is the mass in grams of 15.0 moles of acetaminophen?
135.166 g C8 H 9 NO
3
? g C8 H 9 NO = 15.0 mol C8 H 9 NO
= 2.03 10 g C8H9NO
1 mol C8 H 9 NO
38. For each of the following examples, decide whether it would be better to use the term
molecule or formula unit. (Obj 9)
a. Cl2O molecular compound - molecules
b. Na2O ionic compound formula units
c. (NH4)2SO4
ionic compound formula units
d. HC2H3O2
molecular compound - molecules
40. For each of the following ionic substances, calculate its formula mass and write a conversion
factor that converts between mass in grams and moles of the substance. (Objs 10 & 11)
a. BiBr3
formula mass BiBr3 = 1(208.9804) + 3(79.904)
448.69 g BiBr3
= 448.69 leads to
1 mol BiBr3
b. Al2(SO4)3
formula mass Al2(SO4)3 = 2(26.9815) + 3(32.066) + 12(15.9994)
342.154 g Al 2 (SO 4 )3
= 342.154 leads to
1 mol Al 2 (SO 4 )3
Chapter 9 Chemical Calculations and Chemical Formulas
127
42. A common antacid tablet contains 500 mg of calcium carbonate, CaCO3. (Obj 12)
a. How moles many of CaCO3 does each tablet contain?
1 g 1 mol CaCO3
? mol CaCO3 = 500 mg CaCO3 3
10 mg 100.087 g CaCO3
= 5.00 103 mol CaCO3
b. What is the mass in kilograms of 100.0 moles of calcium carbonate?
100.087 g CaCO3 1 kg
? kg CaCO3 = 100.0 mol CaCO3
3
1 mol CaCO3 10 g
= 10.01 kg CaCO3
44. Rubies and other minerals in the durable corundum family are primarily composed of
aluminum oxide, Al2O3, with trace impurities that lead to their different colors. For example,
the red color in rubies comes from a small amount of chromium replacing some of the
aluminum. If a 0.78-carat ruby were pure aluminum oxide, how many moles of Al2O3 would
be in the stone? (There are exactly 5 carats per gram.) (Obj 12)
1 g 1 mol Al2 O3
3
? mol Al2 O3 = 0.78 carat Al2 O3
= 1.5 10 mol Al2O3
5 carats 101.9612 g Al2 O3
Section 9.4 Relationships Between Masses of Elements and Compounds
46. Write a conversion factor that converts between moles of nitrogen in nitrogen pentoxide,
N2O5, and moles of N2O5. (Obj 13)
2 mol N
1 mol N 2O 5
48. The green granules on older asphalt roofing are chromium(III) oxide. Write a conversion
factor that converts between moles of chromium ions in chromium(III) oxide, Cr2O3, and
moles of Cr2O3. (Objs 13 & 14)
2 mol Cr
1 mol Cr2O 3
50. Ammonium oxalate is used for stain and rust removal. How many moles of ammonium ions
are in one mole of ammonium oxalate, (NH4)2C2O4? (Obj 14)
There are two moles of ammonium ions in one mole of (NH4)2C2O4.
52. A nutritional supplement contains 0.405 g of CaCO3. The recommended daily value of
calcium is 1.000 g Ca. (Objs 13 & 15)
a. Write a conversion factor that relates moles of calcium to moles of calcium carbonate.
1 mol Ca
1 mol CaCO 3
b. Calculate the mass in grams of calcium in 0.405 g of CaCO3.
1 mol CaCO3 1 mol Ca 40.078 g Ca
? g Ca = 0.405 g CaCO3
100.087 g CaCO3 1 mol CaCO3 1 mol Ca
= 0.162 g Ca
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Study Guide for An Introduction to Chemistry
c. What percentage of the daily value of calcium comes from this tablet?
0.162 g Ca in supp.
% Ca =
100 = 16.2% of daily value Ca
1.000 g Ca total
54. A multivitamin tablet contains 10 g of vanadium in the form of sodium metavanadate,
NaVO3. How many micrograms of NaVO3 does each tablet contain? (Obj 15)
1 g 1 mol V 1 mol NaVO3 121.9295 g NaVO3 106 g
? g NaVO3 = 10 g V 6
10 g 50.9415 g V 1 mol V 1 mol NaVO3 1 g
= 24 g NaVO3
56. There are several natural sources of the element titanium. One is the ore called rutile, which
contains oxides of iron and titanium, FeO and TiO2. Titanium metal can be made by first
converting the TiO2 in rutile to TiCl4 by heating the ore to high temperature in the presence
of carbon and chlorine. The titanium in TiCl4 is then reduced from its 4 oxidation state to its
zero oxidation state by reaction with a good reducing agent such as magnesium or sodium.
What is the mass of titanium in kilograms in 0.401 Mg of TiCl4? (Obj 15)
106 g 1 mol TiCl4 1 mol Ti 47.867 g Ti 1 kg
? kg Ti = 0.401 Mg TiCl4
3
1 Mg 189.678 g TiCl4 1 mol TiCl4 1 mol Ti 10 g
= 101 kg Ti
Section 9.5 Determination of Empirical and Molecular Formulas
59. An extremely explosive ionic compound is made from the reaction of silver compounds with
ammonia. A sample of this compound is found to contain 17.261 g of silver and 0.743 g of
nitrogen. What is the empirical formula for this compound? What is its chemical name?
(Obj 16)
1 mol Ag
? mol Ag = 17.261 g Ag
= 0.16002 mol Ag 0.0530 3 mol Ag
107.8682 g Ag
1 mol N
? mol N = 0.743 g N
= 0.0530 mol N 0.0530 = 1 mol N
14.0067 g N
Empirical Formula Ag3N silver nitride
61. A sample of a compound used to polish dentures and as a nutrient and dietary supplement is
analyzed and found to contain 9.2402 g of calcium, 7.2183 g of phosphorus, and 13.0512 g
of oxygen. What is the empirical formula for this compound? (Obj 16)
1 mol Ca
? mol Ca = 9.2402 g Ca
= 0.23056 mol Ca 0.23056 = 1 mol Ca 2 = 2 mol Ca
40.078 g Ca
1 mol P
? mol P = 7.2183 g P
= 0.23305 mol P 0.23056 1 mol P 2 = 2 mol P
30.9738 g P
1 mol O
1
? mol O = 13.0512 g O
= 0.815731 mol O 0.23056 3 mol O 2 = 7 mol O
2
15.9994 g O
Empirical Formula Ca2P2O7
Chapter 9 Chemical Calculations and Chemical Formulas
129
63. An ionic compound that is 38.791% nickel, 33.011% arsenic, and 28.198% oxygen is
employed as a catalyst for hardening fats used to make soap. What is the empirical formula
for this compound? (Obj 17)
1 mol Ni
1
? mol Ni = 38.791 g Ni
= 0.66091 mol Ni 0.44061 = 1 mol Ni 2 = 3 mol Ni
2
58.6934 g Ni
1 mol As
? mol As = 33.011 g As
= 0.44061 mol As 0.44061 = 1 mol As 2 = 2 mol As
74.9216 g As
1 mol O
? mol O = 28.198 g O
= 1.7624 mol O 0.44061 4 mol O 2 = 8 mol O
15.9994 g O
Empirical Formula Ni3As2O8
65. An ionic compound that contains 10.279% calcium, 65.099% iodine, and 24.622% oxygen is
used in deodorants and in mouthwashes. What is the empirical formula for this compound?
(Obj 17)
1 mol Ca
? mol Ca = 10.279 g Ca
= 0.25647 mol Ca 0.25647 = 1 mol Ca
40.078 g Ca
1 mol I
? mol I = 65.099 g I
= 0.51298 mol I 0.25647 2 mol I
126.9045 g I
1 mol O
? mol O = 24.622 g O
= 1.5389 mol O 0.25647 = 6 mol O
15.9994 g O
Empirical Formula CaI2O6 or Ca(IO3)2 calcium iodate
67. In 1989 a controversy arose concerning the chemical daminozide, or Alar, which was
sprayed on apple trees to yield redder, firmer, and more shapely apples. Concerns about
Alars safety stemmed from the suspicion that one of its breakdown products, unsymmetrical
dimethylhydrazine (UDMH), was carcinogenic. Alar is no longer sold for food uses. UDMH
has the empirical formula of CNH4 and has a molecular mass of 60.099. What is the
molecular formula for UDMH? (Obj 17)
Molecular Formula = (Empirical Formula)n
molecular mass
60.099
n=
=
2
empirical formula mass 30.049
Molecular Formula = (CNH4)2 or C2N2H8
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Study Guide for An Introduction to Chemistry
69. Lindane is one of the chlorinated pesticides the use of which is now restricted in the United
States. It is 24.78% carbon, 2.08% hydrogen, and 73.14% chlorine and has a molecular mass
of 290.830. What is lindanes molecular formula? (Obj 18)
1 mol C
? mol C = 24.78 g C
= 2.063 mol C 2.063 = 1 mol C
12.011 g C
1 mol H
? mol H = 2.08 g H
= 2.064 mol H 2.063 1 mol H
1.00794 g H
1 mol Cl
? mol Cl = 73.14 g Cl
= 2.063 mol Cl 2.063 = 1 mol Cl
35.4527 g Cl
Empirical Formula CHCl
molecular mass
290.830
n=
=
6
empirical formula mass 48.472
Molecular Formula = (CHCl)6 or C6H6Cl6
71. Melamine is a compound used to make the melamine-formaldehyde resins in very hard
surface materials such as Formica. It is 28.57% carbon, 4.80% hydrogen, and 66.63%
nitrogen and has a molecular mass of 126.121. What is melamines molecular formula?
(Obj 18)
1 mol C
? mol C = 28.57 g C
= 2.379 mol C 2.379 = 1 mol C
12.011 g C
1 mol H
? mol H = 4.80 g H
= 4.76 mol H 2.379 2 mol H
1.00794 g H
1 mol N
? mol N = 66.63 g N
= 4.757 mol N 2.379 2 mol N
14.0067 g N
Empirical Formula CH2N2
molecular mass
126.121
n=
=
3
empirical formula mass 42.040
Molecular Formula = (CH2N2)3 or C3H6N6
Additional Problems
73. Your boss at the hardware store points you to a bin of screws and asks you to find out the
approximate number of screws it contains. You weigh the screws and find that their total
mass is 68 pounds. You take out 100 screws and weigh them individually, and you find that 7
screws weigh 2.65 g, 4 screws weigh 2.75 g, and 89 screws weigh 2.90 g. Calculate the
weighted average mass of each screw. How many screws are in the bin? How many gross of
screws are in the bin?
weighted average = 0.07(2.65 g) + 0.04(2.75 g) + 0.89(2.90 g) = 2.88 g
453.6 g 1 screw
4
? screws = 68 lb screws
= 1.1 x 10 screws
1 lb 2.88 g
453.6 g 1 screw 1 gross screws
? screws = 68 lb screws
= 74 gross screws
1 lb 2.88 g 144 screws
Chapter 9 Chemical Calculations and Chemical Formulas
131
75. As a member of the corundum family of minerals, sapphire (the September birthstone)
consists primarily of aluminum oxide, Al2O3. Small amounts of iron and titanium give it its
rich dark blue color. Gem cutter Norman Maness carved a giant sapphire into the likeness of
Abraham Lincoln. If this 2302-carat sapphire were pure aluminum oxide, how many moles
of Al2O3 would it contain? (There are exactly 5 carats per gram.)
1 g 1 mol Al2 O3
? mol Al2 O3 = 2302 carat Al2 O3
= 4.515 molAl 2O 3
5 carats 101.9612 g Al2O3
77. Aquamarine (the March birthstone) is a light blue member of the beryl family, which is made
up of natural silicates of beryllium and aluminum that have the general formula
Be3Al2(SiO3)6. Aquamarines bluish color is caused by trace amounts of iron(II) ions. A
43-pound aquamarine mined in Brazil in 1910 remains the largest gem-quality crystal ever
found. If this stone were pure Be3Al2(SiO3)6, how many moles of beryllium would it contain?
453.6 g 1 mol Be3Al2 (SiO3 )6
? mol Be3 Al2 (SiO3 )6 = 43 lb Be3 Al2 (SiO3 )6
1 lb 537.502 g Be3 Al2 (SiO3 )6
3 mol Be
2
= 1.110 mol Be
1 mol Be3Al2 (SiO3 )6
79. Novembers birthstone is citrine, a yellow member of the quartz family. It is primarily silicon
dioxide, but small amounts of iron(III) ions give it its yellow color. A high-quality citrine
containing about 0.040 moles of SiO2 costs around $225. If this stone were pure SiO2, how
many carats would it weigh? (There are exactly 5 carats per gram.)
60.0843 g SiO 2 5 carat
? carat SiO 2 = 0.040 mol SiO 2
= 12 carats
1 mol SiO 2 1 g
81. A common throat lozenge contains 29 mg of phenol, C6H5OH.
a. How many moles of C6H5OH are there in 5.0 mg of phenol?
1 g 1 mol C6 H 5OH
5
? mol C6 H 5OH = 5.0 mg C6 H 5OH 3
= 5.310 mol C6H5OH
10 mg 94.113 g C6 H 5OH
b. What is the mass in kilograms of 0.9265 mole of phenol?
94.113 g C6 H 5OH 1 kg
? kg C6 H 5OH = 0.9265 mol C6 H 5OH
3 = 0.08720 kg C6H5OH
1 mol C6 H 5OH 10 g
83. Beryl, Be3Al2(SiO3)6, is a natural source of beryllium, a known carcinogen. What is the mass
in kilograms of beryllium in 1.006 Mg of Be3Al2(SiO3)6?
106 g 1 mol Be3 Al2 (SiO3 )6
3 mol Be
? kg Be = 1.006 Mg Be3 Al2 (SiO3 )6
1 Mg 537.502 g Be3 Al2 (SiO3 )6 1 mol Be3 Al2 (SiO3 )6
9.0122 g Be 1 kg
3 = 50.60 kg Be
1 mol Be 10 g
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Study Guide for An Introduction to Chemistry
85. Cermets (for ceramic plus metal) are synthetic substances with both ceramic and metallic
components. They combine the strength and toughness of metal with the resistance to heat
and oxidation that ceramics offer. One cermet containing molybdenum and silicon was used
to coat molybdenum engine parts on space vehicles. A sample of this compound is analyzed
and found to contain 14.212 g of molybdenum and 8.321 g of silicon. What is the empirical
formula for this compound?
1 mol Mo
? mol Mo = 14.212 g Mo
= 0.1481 mol Mo 0.1481 = 1 mol Mo
95.94 g Mo
1 mol Si
? mol Si = 8.321 g Si
= 0.2963 mol Si 0.1481 2 mol Si
28.0855 g Si
Empirical Formula MoSi2
87. A compound that is sometimes called sorrel salt can be used to remove ink stains or to clean
wood. It is 30.52% potassium, 0.787% hydrogen, 18.75% carbon, and 49.95% oxygen. What
is the empirical formula for this compound?
1 mol K
? mol K = 30.52 g K
= 0.7806 mol K 0.7806 = 1 mol K
39.0983 g K
1 mol H
? mol H = 0.787 g H
= 0.781 mol H 0.7806 1 mol H
1.00794 g H
1 mol C
? mol C = 18.75 g C
= 1.561 mol C 0.7806 2 mol C
12.011 g C
1 mol O
? mol O = 49.95 g O
= 3.122 mol O 0.7806 4 mol O
15.9994 g O
Empirical Formula KHC2O4
89. An ionic compound that is 24.186% sodium, 33.734% sulfur, and 42.080% oxygen is used as
a food preservative. What is its empirical formula?
1 mol Na
? mol Na = 24.186 g Na
= 1.0520 mol Na 1.0520 = 1 mol Na 2 = 2 mol Na
22.9898 g Na
1 mol S
? mol S = 33.734 g S
= 1.0520 mol S 1.0520 1 mol S 2 = 2 mol S
32.066 g S
1 mol O
1
? mol O = 42.080 g O
= 2.6301 mol O 1.0520 = 2 mol O 2 = 5 mol O
2
15.9994 g O
Empirical Formula Na2S2O5
Chapter 9 Chemical Calculations and Chemical Formulas
133
91. An ionic compound 22.071% manganese, 1.620% hydrogen, 24.887% phosphorus, and
51.422% oxygen is used as a food additive and dietary supplement. What is the empirical
formula for this compound? What do you think its chemical name is? (Consider the
possibility that this compound contains more than one polyatomic ion.)
1 mol Mn
? mol Mn = 22.071 g Mn
= 0.40174 mol Mn 0.40174 = 1 mol Mn
54.9380 g Mn
1 mol H
? mol H = 1.620 g H
= 1.607 mol H 0.40174 4 mol H
1.00794 g H
1 mol P
? mol P = 24.887 g P
= 0.80349 mol P 0.40174 2 mol P
30.9738 g P
1 mol O
? mol O = 51.422 g O
= 3.2140 mol O 0.40174 8 mol O
15.9994 g O
Empirical Formula MnH4P2O8 or Mn(H2PO4)2 manganese(II) dihydrogen phosphate
93. Thalidomide was used as a tranquilizer and flu medicine for pregnant women in Europe until
it was found to cause birth defects. (The horrible effects of this drug played a significant role
in the passage of the Kefauver-Harris Amendment to the Food and Drug Act, requiring that
drugs be proved safe before they are put on the market.) Thalidomide is 60.47% carbon,
3.90% hydrogen, 24.78% oxygen, and 10.85% nitrogen and has a molecular mass of 258.23.
What is the molecular formula for thalidomide?
1 mol C
? mol C = 60.47 g C
= 5.035 mol C 0.7746 = 6.5 mol C 2 = 13 mol C
12.011 g C
1 mol H
? mol H = 3.90 g H
= 3.87 mol H 0.7746 5 mol H 2 = 10 mol H
1.00794 g H
1 mol O
? mol O = 24.78 g O
= 1.549 mol O 0.7746 2 mol O 2 = 4 mol O
15.9994 g O
1 mol N
? mol N = 10.85 g N
= 0.7746 mol N 0.7746 = 1 mol N 2 = 2 mol N
14.0067 g N
Empirical Formula C13H10O4N2
molecular mass
258.23
n=
=
=1
empirical formula mass 258.23
Molecular Formula = C13H10O4N2
134
Study Guide for An Introduction to Chemistry
Challenge Problems
95. Calamine is a naturally occurring zinc silicate that contains the equivalent of 67.5% zinc
oxide, ZnO. (The term calamine also refers to a substance used to make calamine lotion.)
What is the mass, in kilograms, of zinc in 1.347 104 kg of natural calamine that is 67.5%
ZnO?
67.5 kg ZnO 103 g 1 mol ZnO 1 mol Zn
? kg Zn = 1.347 104 kg calamine
100 kg calamine 1 kg 81.39 g ZnO 1 mol ZnO
65.39 g Zn 1 kg
3
3 = 7.30 10 kg Zn
1 mol Zn 10 g
97. Flue dust from the smelting of copper and lead contains As2O3. (Smelting is the heating of a
metal ore until it melts, so that its metallic components can be separated.) When this flue dust
is collected, it contains 90% to 95% As2O3. The arsenic in As2O3 can be reduced to the
element arsenic by reaction with charcoal. What is the maximum mass, in kilograms, of
arsenic that can be formed from 67.3 kg of flue dust that is 93% As2O3?
93 kg As 2O3 103 g 1 mol As 2 O3 2 mol As
? kg As = 67.3 kg flue dust
100 kg flue dust 1 kg 197.841 g As 2 O3 1 mol As 2 O3
74.9216 g As 1 kg
3
1 mol As 10 g
93 kg As 2 O3 2 74.9216 g As
? kg As = 67.3 kg flue dust
= 47 kg As
100 kg flue dust 197.841 g As 2 O3
99. Magnesium metal, which is used to make die-cast auto parts, missiles, and space vehicles, is
obtained by the electrolysis of magnesium chloride. Magnesium hydroxide forms magnesium
chloride when it reacts with hydrochloric acid. There are two common sources of magnesium
hydroxide.
a. Magnesium ions can be precipitated from seawater as magnesium hydroxide,
Mg(OH)2. Each kiloliter of seawater yields about 3.0 kg of the compound. How many
metric tons of magnesium metal can be made from the magnesium hydroxide derived
from 1.0 105 kL of seawater?
or
3
3.0 kg Mg(OH) 2 10 g 1 mol Mg(OH) 2
? t Mg = 1.0 10 kL seawater
1 kL seawater 1 kg 58.3197 g Mg(OH) 2
5
1 mol Mg 24.3050 g Mg 1 t
2
= 1.3 10 t Mg
1 mol Mg(OH) 2 1 mol Mg 106 g
b. Brucite is a natural form of magnesium hydroxide. A typical crude ore containing
brucite is 29% Mg(OH)2. What minimum mass, in metric tons, of this crude ore is
necessary to make 34.78 metric tons of magnesium metal?
106 g 1 mol Mg 1 mol Mg(OH) 2
? t ore = 34.78 t Mg
1 t 24.3050 g Mg 1 mol Mg
58.3197 g Mg(OH) 2 1 t 100 t ore
2
6
= 2.9 10 t ore
1 mol Mg(OH) 2 10 g 29 t Mg(OH) 2
Chapter 9 Chemical Calculations and Chemical Formulas
135
101. The element fluorine can be obtained by the electrolysis of combinations of hydrofluoric
acid and potassium fluoride. These compounds can be made from the calcium fluoride, CaF2,
found in nature as the mineral fluorite. Fluorites commercial name is fluorspar. Crude ores
containing fluorite have 15% to 90% CaF2. What minimum mass, in metric tons, of crude ore is
necessary to make 2.4 metric tons of fluorine if the ore is 72% CaF2?
106 g 1 mol F 1 mol CaF2 78.075 g CaF2 1 t 100 t ore
? t ore = 2.4 t F
6
1 t 18.9984 g F 2 mol F 1 mol CaF2 10 g 72 t CaF2
= 6.8 t ore
103. What mass of baking powder that is 36% NaHCO3 contains 1.0 mole of sodium hydrogen
carbonate?
84.007 g NaHCO3 100 g baking powder
? g baking powder = 1.0 mol NaHCO3
36 g NaHCO3
1 mol NaHCO3
2
= 2.3 10 g baking powder
105. Hafnium metal is used to make control rods in water-cooled nuclear reactors and to make
filaments in light bulbs. The hafnium is found with zirconium in zircon sand, which is
about 1% hafnium(IV) oxide, HfO2. What minimum mass, in metric tons, of zircon sand is
necessary to make 120.5 kg of hafnium metal if the sand is 1.3% HfO2?
103 g 1 mol Hf 1 mol HfO 2 210.49 g HfO 2
? t sand = 120.5 kg Hf
1 kg 178.49 g Hf 1 mol Hf 1 mol HfO 2
1 kg 100 kg sand 1 t
3 = 11 t zircon sand
3
10 g 1.3 kg HfO 2 10 kg
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Study Guide for An Introduction to Chemistry