Chapter 5
Molecules and
Compounds
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Chapter 5
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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.
0.8g 2.0
g 0.16
hydrogen of mass
oxygen of mass Ratio Mass
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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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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
How to Represent Compounds
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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 I Br Cl O F
Tro's "Introductory Chemistry",
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Example—Write Formulas for Each of the
Following Compounds.
• Hematite—Composed of four oxide ions for
every three iron ions.
• Acetone—Each molecule contains six
hydrogen atoms, three carbon atoms, and
one oxygen atom.
Fe3O4
C3H6O
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Chapter 5
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Molecules with
Polyatomic Ions
Mg(NO3)2
Compound called
magnesium nitrate.
Symbol of the polyatomic
ion called nitrate.
Symbol of the polyatomic
ion called sulfate.
CaSO4
Compound called
calcium sulfate.
Implied “1” subscript
on magnesium.
Implied “1” subscript
on calcium.
Parentheses to group two NO3s. No parentheses for one SO4.
10
Molecules with
Polyatomic Ions, Continued
Mg(NO3)2
Compound called
magnesium nitrate.
CaSO4
Compound called
calcium sulfate.
Subscript indicating
two NO3 groups. No subscript indicating
one SO4 group.
Implied “1” subscript
on nitrogen, total 2 N.
Implied “1” subscript
on sulfur, total 1 S.
Stated “3” subscript
on oxygen, total 6 O.
Stated “4” subscript
on oxygen, total 4 O.
Tro's "Introductory Chemistry",
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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
• Practice
• (Hg2)3(PO4)2
1 Mg + 4 C + 6 H + 4 O = 15
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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.
13
Molecular Elements
• Certain elements occur as diatomic molecules.
i.e., two atoms of the same element bonded together
H2
Cl2
Br2
I2
7
7A
N2 O2 F2
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Chapter 5
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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.
16
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
• Aluminum chloride, AlCl3
• Chlorine, Cl2
• Acetone, C3H6O
• Carbon monoxide, CO
• Cobalt, Co
= Atomic element.
= Ionic compound.
= Molecular element.
= Molecular compound.
= Molecular compound.
= 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. 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 total charge of
the cations cancels the total
charge of the anions.
Al+3 column 3A
O2- column 6A
Al+3 O2-
Al2O3
Al = (2)∙(+3) = +6
O = (3)∙(-2) = -6
Tro's "Introductory Chemistry",
Chapter 5
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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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Chapter 5
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Metal Cations
22
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
23
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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Chapter 5
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24
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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25
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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26
1) Naming binary Ionic Compounds for
Metals with Invariant Charge
2) Naming Binary Ionic Compounds for
Metals with Variable Charge
Su
mm
ary
Su
mm
ary
S
um
mary
Tro's "Introductory Chemistry",
Chapter 5
27
Example : What is the name of the
compound CsF
27
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
Tro's "Introductory Chemistry",
Chapter 5
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Practice: Name the following compounds
28
1. KCl
2. MgBr2
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Chapter 5
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29
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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Chapter 5
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Practice : Find the charge on the cation.
30
CrO3
Fe3N2
Tro's "Introductory Chemistry",
Chapter 5
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31
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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Chapter 5
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32
Practice: Name the following compounds.
1. TiCl4
2. Fe2S3
Tro's "Introductory Chemistry",
Chapter 5
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33
Example: Writing Formula for Binary Ionic
Compounds Containing Variable Charge Metal
manganese(IV) sulfide
1. Write the symbol for the cation
and its charge
2. Write the symbol for the anion
and its charge
3. Switch charge (without sign)
each ion to become subscript
for the other ion
4. Reduce subscripts to smallest
whole number ratio
5. Check that the total charge of
the cations cancels the total
charge of the anions
Mn4+
S2–
Mn4+ S2− Mn2S4
Mn = (1) × (4+) = +4
S = (2) × (2–) = −4
MnS2
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Chapter 5
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34
Practice: What are the formulas for
compounds made from the following ions?
1) copper(II) nitride
2) iron(III) bromide
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Chapter 5
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35
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
Some Common Polyatomic Ions
Name Formula
acetate C2H3O2–
carbonate CO32–
hydrogen carbonate HCO3–
hydroxide OH–
nitrate NO3–
nitrite NO2–
chromate CrO42–
dichromate Cr2O72–
cyanide CN–
permanganate MnO4–
ammonium NH4+
Name Formula
hypochlorite ClO–
chlorite ClO2–
chlorate ClO3–
perchlorate ClO4–
sulfate SO42–
sulfite SO32–
hydrogen sulfate HSO4–
hydrogen sulfite HSO3–
phosphate PO43–
phosphite PO33–
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Chapter 5
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37
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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38
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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Chapter 5
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39
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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40
Example: Writing Formula for Ionic
Compounds Containing Polyatomic Ion
Iron(III) phosphate
1. Write the symbol for the cation
and its charge
2. Write the symbol for the anion
and its charge
3. Switch the charge (without
sign) of each ion to become
subscript for the other ion
4. Reduce subscripts to smallest
whole number ratio.
5. Check that the total charge of
the cations cancels the total
charge of the anions
Fe3+
PO43−
Fe3+ PO43− Fe3(PO4)3
Fe = (1) × (3+) = +3
PO4 = (1) × (3−) = −3
FePO4
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Chapter 5
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41
Practice: Write the chemical formula of the
following compounds?
aluminum sulfate
chromium(II) hydrogencarbonate
Tro's "Introductory Chemistry",
Chapter 5
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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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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.
Example—Naming Ionic with Polyatomic Ion,
Na2SO4 , Continued
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44
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
Tro's "Introductory Chemistry",
Chapter 5
45
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.
Example—Naming Ionic with Polyatomic Ion,
Fe(NO3)3 , Continued
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Chapter 5
46
Practice─Name the Following
1. NH4Cl
2. Ca(C2H3O2)2
3. Cu(NO3)2
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Chapter 5
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47
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
48
• 1 = mono-
not used on first nonmetal
• 2 = di-
• 3 = tri-
• 4 = tetra-
• 5 = penta-
• 6 = hexa-
• 7 = hepta-
• 8 = octa-
• 9 = nona-
• 10 = deca-
• Drop last “a” or “o” if name begins with a vowel.
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49
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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Chapter 5
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50
Practice: Name the following.
N2O4
PCl5
I2F7
CO
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Chapter 5
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51
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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Chapter 5
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52
Practice: Write formulas for the following.
dinitrogen tetroxide
sulfur hexafluoride
diarsenic trisulfide
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Chapter 5
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53
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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54
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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55
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
Naming Binary Acids
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Chapter 5
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56
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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Chapter 5
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57
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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Chapter 5
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58
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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Chapter 5
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59
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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Chapter 5
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60
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
3. Write the name of the anion followed by the word acid.
sulfuric acid
sulfuric
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Chapter 5
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61
Practice: Name the following
HBr (aq)
HClO3(aq)
H3PO4(aq)
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62
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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63
Example: Binary Acids
hydrobromic acid
1. Write the symbol for the cation
and its charge.
2. Write the symbol for the anion
and its charge.
3. Switch charges (without sign) for
each ion to subscript for the other
ion.
4. Add (aq) to indicate dissolved in
water.
H+
Br−
H+ Br− HBr
HBr(aq)
In all acids, the
cation is H+.
Hydro means
binary.
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Chapter 5
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64
Example: Oxyacids
carbonic acid
1. Write the symbol for the cation
and its charge
2. Write the symbol for the anion
and its charge
3. Switch charge (without sign) for
each ion to become subscript for
the other ion
4. Add (aq) to indicate dissolved in
water
H+
CO32−
H+ CO32− H2CO3
H2CO3(aq)
In all acids, the
cation is H+.
No hydro means
polyatomic ion.
-ic means -ate ion.
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Chapter 5
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65
Example: Oxyacids
sulfurous acid
1. Write the symbol for the cation
and its charge.
2. Write the symbol for the anion
and its charge.
3. Switch charge (without sign) of
each ion to become subscript
for the other ion
4. Add (aq) to indicate dissolved
in water.
H+
SO32−
H+ SO32− H2SO3
H2SO3(aq)
In all acids, the
cation is H+.
No hydro means
polyatomic ion.
-ous means -ite ion.
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Chapter 5
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66
Practice: What are the formulas for the
following acids?
chlorous acid
phosphoric acid
hydrobromic acid
Compounds With Trivial Names
Name Formula
Water H2O
Methane CH4
Ammonia NH3
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Chapter 5
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67
Tro's "Introductory Chemistry",
Chapter 5
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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
amu 17.342)(SOAl
amu 16.00 12O
amu 32.07 3S
amu 26.98 2Al
342
Tro's "Introductory Chemistry",
Chapter 5
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Practice—Calculate the Formula Mass of
Fe2(CO3)3
Answer = 291.73 amu
Tro's "Introductory Chemistry",
Chapter 5
72
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