Transcript Introductory Chemistry, 2nd Edition Nivaldo Tro

Chapter 5
Molecules and
Compounds
Molecules and Compounds
• When elements combine to form
compounds their properties change
• Salt
 Sodium—shiny, reactive,
poisonous.
 Chlorine—pale yellow gas, reactive,
poisonous.
 Sodium chloride—table salt.
• Sugar
 Carbon—pencil or diamonds.
 Hydrogen—flammable gas.
 Oxygen—a gas in air.
 Combine to form white crystalline
sugar.
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Compounds Display Constant
Composition
• All pure substances have constant composition.
All samples of a pure substance contain the same
elements in the same percentages (ratios).
Mixtures have variable composition.
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Example: Constant Composition
If we decompose water, we find 16.0 grams of
oxygen to every 2.00 grams of hydrogen.
Water has a constant mass ratio of oxygen to
hydrogen of 8.0.
mass of oxygen
Mass Ratio 
mass of hydrogen
16.0 g

 8.0
2.0 g
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How to Represent Compounds
• A compound is substance that is composed of atoms
of two or more elements.
• Compounds can be represented
 Writing the symbol of each element present in the
compound.
 describing the number of each atom in the simplest unit of
the compound.
• The number of atoms of each element is written to the
right of the element as a subscript.
 If there is only one atom, the 1 subscript is not written.
• Polyatomic groups are placed in parentheses.
 If more than one.
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How to Represent Compounds
Water = H2O \ two atoms of
hydrogen and 1 atom of oxygen
Table sugar = C12H22O11 \12 atoms
of C, 22 atoms of H and 11 atoms O
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Order of Elements in a Formula
• Metals are written first.
 NaCl
• Nonmetals are written in order from
Table 5.1.
 CO2
 There are occasional exceptions for
historical or informational reasons.
 H2O, but NaOH .
Table 5.1
Order of Listing Nonmetals
in Chemical Formulas
C P N H S
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Example—Write Formulas for Each of the
Following Compounds.
• Hematite—Composed of four oxide ions for
every three iron ions. Fe O
3
4
• Acetone—Each molecule contains six
hydrogen atoms, three carbon atoms, and
one oxygen atom.
C3H6O
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Molecules with
Polyatomic Ions
Symbol of the polyatomic
ion called nitrate.
Symbol of the polyatomic
ion called sulfate.
Mg(NO3)2
CaSO4
Compound called
magnesium nitrate.
Compound called
calcium sulfate.
Implied “1” subscript
on magnesium.
Parentheses to group two NO3s.
Implied “1” subscript
on calcium.
No parentheses for one SO4.
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Molecules with
Polyatomic Ions, Continued
Subscript indicating
two NO3 groups.
No subscript indicating
one SO4 group.
Mg(NO3)2
CaSO4
Compound called
magnesium nitrate.
Compound called
calcium sulfate.
Implied “1” subscript
on nitrogen, total 2 N.
Stated “3” subscript
on oxygen, total 6 O.
Implied “1” subscript
on sulfur, total 1 S.
Stated “4” subscript
on oxygen, total 4 O.
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Example—Determine the Total Number of Atoms
or Ions in One Formula Unit of Each of the
Following.
• Mg(C2H3O2)2 1 Mg + 4 C + 6 H + 4 O = 15
• Practice
• (Hg2)3(PO4)2
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Classifying Materials
• Atomic elements = Elements whose
particles are single atoms.
• Molecular elements = Elements whose
particles are multi-atom molecules.
• Molecular compounds = Compounds
whose particles are molecules made of only
nonmetals.
• Ionic compounds = Compounds whose
particles are cations and anions.
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Molecular Elements
• Certain elements occur as diatomic molecules.
i.e., two atoms of the same element bonded together
7A
H2
N2
7
O2
F2
Cl2
Br2
I2
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Molecular Compounds
• Two or more
nonmetals.
• Smallest unit is a
molecule.
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Ionic Compounds
• Metals + nonmetals.
• No individual
molecule units, instead
have a 3-dimensional
array of cations and
anions made of
formula units.
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Molecular View of
Elements and Compounds
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Classify Each of the Following as Either an
Atomic Element, Molecular Element,
Molecular Compound, or Ionic Compound,
Continued.
• Aluminum, Al = Atomic element.
• Aluminum chloride, AlCl3 = Ionic compound.
• Chlorine, Cl2 = Molecular element.
• Acetone, C3H6O = Molecular compound.
• Carbon monoxide, CO = Molecular compound.
• Cobalt, Co = Atomic element
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Ionic Compounds
• Ionic compounds are made of ions.
• Ionic compounds always contain cations and
anions.
Cations = + charged ions; anions = − charged ions.
• The sum of the + charges of the cations must
equal the sum of the − charges of the anions.
• If Na+ is combined with S2-, you will need 2 Na+
ions for every S2- ion to balance the charges,
therefore the formula must be Na2S.
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Writing Formulas for
Ionic Compounds
NAME-TO-FORMULA
1. Write the symbol for the metal cation and its charge.
2. Write the symbol for the nonmetal anion and its charge.
3. Charge (without sign) becomes subscript for the other
ion.
4. Reduce subscripts to smallest whole-number ratio.
5. Check that the sum of the charges of the cation cancels
the sum of the anions.
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Write the Formula of a Compound Made from
Aluminum Ions and Oxide Ions.
1.
2.
3.
4.
5.
Write the symbol for the metal
cation and its charge.
Write the symbol for the
nonmetal anion and its charge.
Charge (without sign) becomes
subscript for the other ion.
Reduce subscripts to smallest
whole-number ratio.
Check that the total charge of
the cations cancels the total
charge of the anions.
Al+3 column 3A
O2- column 6A
Al+3 O2Al2O3
Al = (2)∙(+3) = +6
O = (3)∙(-2) = -6
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Practice—What Are the Formulas for
Compounds Made from the Following
Ions?
• Potassium ion with a nitride ion.
• Calcium ion with a bromide ion.
• Aluminum ion with a sulfide ion.
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Metal Cations
TYPE 1
• Main group metals
have invariant charge i.e.,
metals whose ions can
have only one possible
charge
Groups 1A & 2A,
Al3+, Ag1+, Zn2+, Sc3+
cation name = metal
name
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Metal Cations
TYPE 2
• Transition metals
 metals with variable
charges i.e., metals
whose ions can have
more than one possible
charge
 determine charge of
cation by charge on
anion
 name = metal name
with Roman numeral
charge in parentheses
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Naming Monatomic Nonmetal Anion
• Determine the charge from position on the
Periodic Table.
• To name anion, change ending on the element
name to -ide.
4A = 4−
5A = 3−
6A = 2−
7A = 1−
C = carbide
N = nitride
O = oxide
F = fluoride
Si = silicide
P = phosphide
S = sulfide Cl = chloride
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Naming Ionic Compounds
FORMULA-TO-NAME
• Recall: ionic compounds are made of cations and
anions
• Name the cation first, followed by the anion
if cation is:
main group metal = metal name
transition metal = metal name(charge)
polyatomic ion = name of polyatomic ion
if anion is:
nonmetal = stem of nonmetal name + -ide
polyatomic ion = name of polyatomic ion
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Summary
Summary
Summary
1) Naming binary Ionic Compounds for
Metals with Invariant Charge
2) Naming Binary Ionic Compounds for
Metals with Variable Charge
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Example : What is the name of the
compound CsF
1. Identify cation and anion.
Cs = Cs+ because it is in Group 1A
F = F– because it is in Group 7A
2. Name the cation.
Cs+ = cesium
3. Name the anion.
F– = fluoride
4. Write the cation name first, then the anion name.
cesium fluoride
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Practice: Name the following compounds
1.
KCl
2.
MgBr2
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Determining the Charge on a Cation with
Variable Charge e.g., Au2S3
1.
Determine the charge on the anion
Au2S3—The anion is S. Since it is in Group 6A, its
charge is 2−
2.
Determine the total negative charge
Since there are 3 S in the formula, the total negative
charge is −6
3.
Determine the total positive charge
Since the total negative charge is −6, the total positive
charge is +6
4.
Divide by the number of cations
Since there are 2 Au in the formula and the total positive
charge is +6, each Au has a 3+ charge
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Practice : Find the charge on the cation.
CrO3
Fe3N2
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Example: Naming Binary Ionic
with Variable Charge Metal CuF2
1. Identify cation and anion
F = F− because it is Group 7
Cu = Cu2+ to balance the two (−) charges from 2 F−
2. Name the cation
Cu2+ = copper(II)
3. Name the anion.
F− = fluoride
4. Write the cation name first, then the anion name.
copper(II) fluoride
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Practice: Name the following compounds.
1.
TiCl4
2.
Fe2S3
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Example: Writing Formula for Binary Ionic
Compounds Containing Variable Charge Metal
manganese(IV) sulfide
1.
2.
3.
4.
5.
Write the symbol for the cation
4+
Mn
and its charge
Write the symbol for the anion
S2–
and its charge
Switch charge (without sign)
Mn4+ S2− Mn2S4
each ion to become subscript
for the other ion
Reduce subscripts to smallest
MnS2
whole number ratio
Check that the total charge of
Mn = (1) × (4+) = +4
the cations cancels the total
S = (2) × (2–) = −4
charge of the anionsTro's "Introductory Chemistry",
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Chapter 5
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Practice: What are the formulas for
compounds made from the following ions?
1) copper(II) nitride
2) iron(III) bromide
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Compounds Containing
Polyatomic Ions
• Polyatomic ions are single ions that contain
more than one atom
• Name and charge of polyatomic ion do not
change
• As usual, name any ionic compound by naming
cation first and then anion
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Some Common Polyatomic Ions
Name
Formula
Name
Formula
acetate
carbonate
hydrogen carbonate
C2H3O2–
CO32–
HCO3–
hypochlorite
chlorite
chlorate
ClO–
ClO2–
ClO3–
hydroxide
nitrate
nitrite
chromate
OH–
NO3–
NO2–
CrO42–
perchlorate
sulfate
sulfite
hydrogen sulfate
ClO4–
SO42–
SO32–
HSO4–
dichromate
Cr2O72–
hydrogen sulfite
HSO3–
cyanide
permanganate
CN–
MnO4–
phosphate
phosphite
PO43–
PO33–
ammonium
NH4+
Patterns for Polyatomic Ions
1. The ‘ate ’ and the ‘ite’ ions. Ions with same
charge but one less oxygen atom.
NO3– Nitrate
NO2– Nitrite
2. hydrogen- prefix before name and add 1 to the
charge
CO32– = carbonate \ HCO3– = hydrogen carbonate
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Periodic Pattern of Polyatomic Ions
-ate groups
3A
4A
5A
6A
7A
BO33−
borate
CO32−
carbonate
NO3−
nitrate
SiO32−
silicate
PO43−
phosphate
SO42−
sulfate
ClO3−
chlorate
AsO43−
arsenate
SeO42−
selenate
BrO3−
bromate
TeO42−
tellurate
IO3−
iodate
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Patterns for Polyatomic Ions
• -ate ion
chlorate = ClO3–
• -ate ion + 1 O  same charge, per- prefix
perchlorate = ClO4–
• -ate ion – 1 O  same charge, -ite suffix
chlorite = ClO2–
• -ate ion – 2 O  same charge, hypo- prefix, -ite
suffix
hypochlorite = ClO–
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Example: Writing Formula for Ionic
Compounds Containing Polyatomic Ion
Iron(III) phosphate
1.
2.
3.
4.
5.
Write the symbol for the cation
3+
Fe
and its charge
Write the symbol for the anion
PO43−
and its charge
Switch the charge (without
Fe3+ PO43− Fe3(PO4)3
sign) of each ion to become
subscript for the other ion
Reduce subscripts to smallest
FePO4
whole number ratio.
Check that the total charge of
Fe = (1) × (3+) = +3
the cations cancels the total
PO4 = (1) × (3−) = −3
charge of the anionsTro's "Introductory Chemistry",
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Chapter 5
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Practice: Write the chemical formula of the
following compounds?
aluminum sulfate
chromium(II) hydrogencarbonate
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Example—Naming Ionic with Polyatomic Ion,
Na2SO4
1. Identify cation
Na is the cation because metals form positive charge.
•
Is the metal Type I or Type II?
Na is in Group 1A, \ Type I.
Hence, charge on Na is +1
•
Identify anion
Anion is SO4
Polyatomic, because it contains TWO OR MORE different types
of elements
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Example—Naming Ionic with Polyatomic Ion,
Na2SO4 , Continued
4. Name the cation.
Na+ = sodium (Type I).
5. Name the anion.
SO42- = sulfate.
6. Write the name of the cation followed by the
name of the anion.
sodium sulfate.
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Example—Naming Ionic with Polyatomic Ion,
Fe(NO3)3
1. Identify cation
Fe is the cation because metals form positive charge.
•
•
Is the metal Type I or Type II?
Fe is a transition metal, \ Type II.
Caution!!: Transition metals have variable charge, so we need to
find out what the charge on the anion
Identify anion
Anion is NO3
polyatomic because it contains TWO OR MORE different types
of elements
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Example—Naming Ionic with Polyatomic Ion,
Fe(NO3)3 , Continued
4. Name the anion.
NO3- = nitrate
5. Name the cation.
Fe3+ = iron(III) (Type II).
Based on the charge of the nitrate group
6. Write the name of the cation followed by the
name of the anion.
iron(III) nitrate.
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Practice─Name the Following
1. NH4Cl
2. Ca(C2H3O2)2
3. Cu(NO3)2
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Writing Names of Binary Molecular
Compounds of Two Nonmetals
1. Write the name of the first element in the formula
 Use the full name of the element,
2. Write the name of the second element in the formula
with an -ide suffix
 as if it were an anion—However, remember these
compounds do not contain ions!
3. Use a prefix in front of each name to indicate the
number of atoms
a) Never use the prefix mono- on the first element.
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Subscript—Prefixes
• 6 = hexa-
• 1 = mono not used on first nonmetal
•
•
•
•
•
•
•
•
2 = di3 = tri4 = tetra5 = penta-
•
Drop last “a” or “o” if name begins with a vowel.
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7 = hepta8 = octa9 = nona10 = deca-
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Example: Naming Binary Molecular.
e.g., BF3
1. Name the first element.
boron
2. Name the second element with an -ide.
fluorine  fluoride
3. Add a prefix to each name to indicate the
subscript.
monoboron trifluoride
 Drop the prefix mono from the first element.
boron trifluoride
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Practice: Name the following.
N 2O 4
PCl5
I2F7
CO
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Example: Binary Molecular
dinitrogen pentoxide
• Identify the symbols of the elements.
nitrogen = N
oxide = oxygen = O
• Write the formula using prefix number for
subscript.
di = 2, penta = 5
N2O5
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Practice: Write formulas for the following.
dinitrogen tetroxide
sulfur hexafluoride
diarsenic trisulfide
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Acids
• Acids are molecular compounds that form H+
when dissolved in water.
To indicate the compound is dissolved in water, (aq)
is written after the formula.
not named as acid if not dissolved in water
• sour taste (please never taste it!!)
• dissolve many metals
like Zn, Fe, Mg; but not Au, Ag, Pt
• formula generally starts with H
e.g., HCl, H2SO4
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Acids
• Dissolve in aqueous
solution to form H+
cation and anion
• Binary acids have H+
cation and nonmetal
anion
• Oxyacids have H+ cation
and polyatomic anion
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Naming Binary Acids
1. Identify the anion
2. Name the anion with an -ic suffix
3. Add a hydro- prefix to the anion name
4. Add the word acid to the end
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Example: Naming Binary
Acids. e.g., HCl(aq)
1. Identify the anion
Cl = Cl−, chloride because Group 7A
2. Name the anion with an -ic suffix
Cl− = chloride  chloric
3. Add a hydro- prefix to the anion name
hydrochloric
4. Add the word acid to the end
hydrochloric acid
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Naming Oxyacids
1) Identify the polyatomic ion (anion)
 The poly atomic ion in this case will always
contain oxygen
2) If the polyatomic ion name ends in -ate, then
change ending to -ic suffix
3) If the polyatomic ion name ends in -ite, then
change ending to -ous suffix
4) Write word acid at the end of all names.
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Example:
Naming Oxyacids. HNO3(aq)
1. Identify the polyatomic ion
NO3 = NO3− = nitrate
2. If the anion has -ate suffix, change it to -ic. If the
anion has -ite suffix, change it to -ous.
NO3− = nitrate  nitric
3. Write the name of the anion followed by the
word acid.
nitric acid
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Example:
Naming Oxyacids. HNO2(aq)
1. Identify the polyatomic ion
NO2 = NO2− = nitrite
2. If the anion has -ate suffix, change it to -ic. If the
anion has -ite suffix, change it to -ous.
NO2− = nitrite  nitrous
3. Write the name of the anion followed by the
word acid.
nitrous acid
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Example: Naming Oxyacids
H2SO4(aq)
1. Identify the anion
SO4 = SO42− = sulfate
2. If the anion has -ate suffix, change it to -ic. If
the anion has -ite suffix, change it to -ous
SO42− = sulfate  sulfuric
3. Write the name of the anion followed by the
word acid.
sulfuric acid
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Practice: Name the following
HBr (aq)
HClO3(aq)
H3PO4(aq)
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Writing Formulas for Acids
1) when name ends in acid, formula starts with H
2) Hydro prefix means it is a binary acid, no prefix means it is an
oxyacid
3) Locate the cation and the anion

NB: In acids the cation is always H+
4) for oxyacid, if ending is -ic, polyatomic ion ends in -ate; if ending
is -ous, polyatomic ion ends in –ite
5) write formulas as if ionic,
even though it is molecular
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Example: Binary Acids
hydrobromic acid
1.
2.
3.
4.
Write the symbol for the cation
and its charge.
Write the symbol for the anion
and its charge.
Switch charges (without sign) for
each ion to subscript for the other
ion.
Add (aq) to indicate dissolved in
water.
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H+
Br−
In all acids, the
cation is H+.
Hydro means
binary.
H+ Br−
HBr
HBr(aq)
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Example: Oxyacids
carbonic acid
1.
2.
3.
4.
Write the symbol for the cation
and its charge
Write the symbol for the anion
and its charge
Switch charge (without sign) for
each ion to become subscript for
the other ion
Add (aq) to indicate dissolved in
water
In all acids, the
cation is H+.
H+
No hydro means
2−
CO3 polyatomic ion.
-ic means -ate ion.
H+ CO32− H2CO3
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H2CO3(aq)
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Example: Oxyacids
sulfurous acid
1.
2.
3.
4.
Write the symbol for the cation
and its charge.
Write the symbol for the anion
and its charge.
Switch charge (without sign) of
each ion to become subscript
for the other ion
Add (aq) to indicate dissolved
in water.
In all acids, the
cation is H+.
H+
No hydro means
SO 2− polyatomic ion.
3
-ous means -ite ion.
H+ SO32−
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H2SO3
H2SO3(aq)
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Practice: What are the formulas for the
following acids?
chlorous acid
phosphoric acid
hydrobromic acid
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Compounds With Trivial Names
Name
Formula
Water
H2O
Methane
CH4
Ammonia
NH3
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Formula-to-Name Flowchart
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Formula Mass
• The mass of an individual molecule or formula
unit.
• Also known as molecular mass or molecular
weight.
• Sum of the masses of the atoms in a single
molecule or formula unit.
 Whole = Sum of the parts.
Mass of 1 molecule of H2O
= 2(1.01 amu H) + 16.00 amu O = 18.02 amu.
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Example—Calculate the Formula Mass of
Al2(SO4)3
Al
S
O
Al2 (SO 4 )3
 2  26.98 amu
 3  32.07 amu
 12  16.00 amu

342.17 amu
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Practice—Calculate the Formula Mass of
Fe2(CO3)3
Answer = 291.73 amu
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Recommended Study Problems Chapter 5
NB: Study problems are used to check the student’s understanding
of the lecture material. Students are EXPECTED TO BE ABLE
TO SOLVE ALL THE SUGGESTED STUDY PROBLEMS.
If you encounter any problems, please talk to your professor or seek
help at the HACC-Gettysburg learning center.
Questions from text book Chapter 5, p 151
23, 25, 29, 31, 33, 35, 39, 41, 49, 51, 55, 57, 61, 63, 67, 69, 71-78
ANSWERS
-The answers to the odd-numbered study problems are found at
the back of your textbook
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