John C. Kotz
Paul M. Treichel
John Townsend
http://academic.cengage.com/kotz
Chapter 2
Atoms, Molecules, and Ions
John C. Kotz • State University of New York, College at Oneonta
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ATOMS, Molecules, & Ions
© 2009 Brooks/Cole - Cengage
3
ATOMIC COMPOSITION
• Protons
– + electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic
mass units (u)
• Electrons
–
negative electrical charge
– relative mass = 0.0005 u
• Neutrons
no electrical charge
– mass = 1.009 u
–
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5
ATOM
COMPOSITION
The atom is mostly
empty space
• protons and neutrons in
the nucleus.
PLAY MOVIE
• the number of electrons is equal to the
number of protons.
• electrons in space around the nucleus.
• extremely small. One teaspoon of water
has 3 times as many atoms as the Atlantic
Ocean has teaspoons of water.
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Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
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Atomic number, Z
Atom symbol
Atomic weight
Atomic Weight
• This tells us the mass of one atom of
an element relative to one atom of
another element.
• OR — the mass of 1000 atoms of one
relative to 1000 atoms of another.
• For example, an O atom is
approximately 16 times heavier than an
H atom.
• Define one element as the standard
against which all others are measured
• Standard = carbon
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Mass Number, A
• C atom with 6 protons and 6 neutrons
is the mass standard
• = 12 atomic mass units (u)
• Mass Number (A)
= # protons + # neutrons
• A boron atom can have
A = 5 p + 5 n = 10 u
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A
10
Z
5
B
8
Boron in Death Valley
• Death Valley has been a major source of
borax and other boron-containing
minerals.
• Borax was transported out of Death Valley
in wagons pulled by teams of 20 mules.
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Isotopes
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 has 5 p and 5 n: 105B
• Boron-11 has 5 p and 6 n: 115B
11B
10B
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10
Hydrogen Isotopes
Hydrogen has _____ isotopes
1 H
1
1 proton and 0
neutrons, protium
2 H
1
1 proton and 1
neutron, deuterium
3 H
1
1 proton and 2
neutrons, tritium
radioactive
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12
Isotope Composition
Isotope
Sulfur-32
Bromine79
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Electrons
Protons
Neutrons
Isotopes &
Their Uses
Heart scans with
radioactive
technetium-99.
99
43Tc
Emits gamma rays
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Masses of Isotopes
determined with a mass spectrometer
See Active Figure 2.3
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Mass spectrum of C6H5Br
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16
11B
Isotopes
10B
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• Average mass = ATOMIC WEIGHT
• Boron is 19.9% 10B and 80.1% 11B. That is, 11B
is 80.1 percent abundant on earth.
• For boron atomic weight
= 0.199 (10.0 u) + 0.801 (11.0 u) = 10.8 u
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Isotopes & Atomic Weight
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• 6Li = 7.5% abundant and 7Li = 92.5%
–Atomic weight of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
–Atomic weight of Si = ______________
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Periodic Table
• Dmitri Mendeleev (18341907) developed the modern
periodic table. Argued that
element properties are
periodic functions of their
atomic weights.
• We now know that element
properties are periodic
functions of their ATOMIC
NUMBERS.
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19
Periods in the Periodic Table
PLAY MOVIE
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Groups/Families in the
Periodic Table
PLAY MOVIE
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Regions of the Periodic Table
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Element Abundance
C
O
Al
Si
http://www.webelements.com/
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Fe
22
Hydrogen
PLAY MOVIE
Shuttle main engines
use H2 and O2
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Group 1A: Alkali Metals
Li, Na, K, Rb, Cs
PLAY MOVIE
Reaction of
potassium + H2O
Cutting sodium metal
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Group 2A: Alkaline Earth Metals
Be, Mg, Ca, Sr, Ba, Ra
Magnesium
Magnesium
oxide
PLAY MOVIE
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Group 3A: B, Al, Ga, In, Tl
Cu
Al
Al resists corrosion
(here in nitric acid).
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Gallium is one of the few
metals that can be liquid
at room temp.
26
Gems & Minerals
• Sapphire: Al2O3
with Fe3+ or Ti3+
impurity gives
blue whereas V3+
gives violet.
• Ruby: Al2O3 with
Cr3+ impurity
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28
Group 4A: C, Si, Ge, Sn, Pb
Quartz, SiO2
Diamond
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Group 5A: N, P, As, Sb, Bi
PLAY MOVIE
Ammonia, NH3
White and red
phosphorus
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Phosphorus
• Phosphorus first isolated by Brandt from urine, 1669
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30
Group 6A: O, S, Se, Te, Po
Sulfuric acid dripping from
snot-tite in cave in Mexico
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Elemental S has a ring
structure.
31
Group 7A:
Halogens
F, Cl, Br, I, At
PLAY MOVIE
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Group 8A: Noble Gases
He, Ne, Ar, Kr, Xe, Rn
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34
Transition Elements
Lanthanides and actinides
Iron in air gives
iron(III) oxide
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Colors of Transition Metal
Compounds
Iron
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Cobalt
Nickel
Copper
Zinc
35
Molecules, Ions &
Their Compounds
PLAY MOVIE
NaCl, salt
Ethanol, C2H6O
Buckyball, C60
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37
Compounds & Molecules
• COMPOUNDS are a combination of 2 or
more elements in definite ratios by mass.
• The character of each element is lost when
forming a compound.
• MOLECULES are the smallest unit of a
compound that retains the characteristics of
the compound.
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38
MOLECULAR FORMULAS
• Formula for glycine is C2H5NO2
• In one molecule there are
– 2 C atoms
– 5 H atoms
– 1 N atom
– 2 O atoms
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WRITING FORMULAS
• Can also write glycine formula as
–H2NCH2COOH
to show atom ordering
• or in the form of a structural
H H O
H N C C O H
H
© 2009 Brooks/Cole - Cengage
formula
40
MOLECULAR MODELING
H H O
H N C C O H
H
Ball & stick
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Structural formula
of glycine
Space-filling
41
Molecular & Ionic Compounds
Fe
N
Iron-porphyrin
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NaCl
42
ELEMENTS THAT EXIST AS
MOLECULES
Allotropes of C
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ELEMENTS THAT EXIST AS
DIATOMIC MOLECULES
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43
ELEMENTS THAT EXIST AS
POLYATOMIC MOLECULES
44
S8 sulfur
molecules
White P4 and polymeric
red phosphorus
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IONS AND IONIC COMPOUNDS
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46
IONS AND IONIC COMPOUNDS
• IONS are atoms or groups of atoms with a positive
or negative charge.
• Taking away an electron from an atom gives a
CATION with a positive charge
• Adding an electron to an atom gives an ANION
with a negative charge.
© 2009 Brooks/Cole - Cengage
Forming Cations & Anions
A CATION forms
when an atom
loses one or
more electrons.
PLAY MOVIE
Mg f
Mg2+
© 2009 Brooks/Cole - Cengage
+ 2 e-
An ANION forms
when an atom
gains one or
more electrons
PLAY MOVIE
F + e- f F-
47
48
See Active Figure 2.17
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PREDICTING ION CHARGES
See Figure 2.18
In general
• metals (Mg) lose electrons f cations
• nonmetals (F) gain electrons f anions
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Charges on Common Ions
-4 -3 -2 -1
+1
+2
+3
By losing or gaining e-, atom has same
number of e-’s as nearest Group 8A atom.
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Predicting Charges on Monatomic Ions
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51
METALS
M f n e- + Mn+
where n = periodic group
Na+
sodium ion
Mg2+
magnesium ion
Al3+
aluminum ion
Transition metals f M2+ or M3+
are common
Fe2+
iron(II) ion
Fe3+
iron(III) ion
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53
NONMETALS
NONMETAL + n e- f Xnwhere n = 8 - Group no.
Group 4A
Group 5A
Group 6A
Group 7A
C4-,carbide N3-, nitride O2-, oxide F-, fluoride
Name derived
by adding -ide
to stem
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S2-, sulfide Cl-, chloride
Br-, bromide
I-, iodide
54
Ion Formation
Reaction of
aluminum
and
bromine
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
POLYATOMIC IONS
Groups of atoms with a
charge.
MEMORIZE the names and
formulas in Table 2.4, page
74.
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Celestite, SrSO4
55
56
Note: many O
containing anions
have names ending
in –ate (or -ite).
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57
Polyatomic Ions
HNO3
nitric acid
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NO3nitrate ion
58
Polyatomic Ions
NH4+
ammonium ion
One of the few common
polyatomic cations
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59
Polyatomic Ions
CO32carbonate ion
HCO3bicarbonate ion
hydrogen carbonate
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Polyatomic Ions
PO43phosphate ion
CH3CO2acetate ion
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61
Polyatomic Ions
SO42sulfate ion
SO32sulfite ion
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Polyatomic Ions
NO3nitrate ion
NO2nitrite ion
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62
COMPOUNDS
FORMED FROM
IONS
63
CATION +
ANION f
COMPOUND
Na+ + Cl- f NaCl
A neutral compd.
requires
equal number of +
and - charges.
© 2009 Brooks/Cole - Cengage
IONIC COMPOUNDS
NH4
+
Cl
ammonium chloride, NH4Cl
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64
Some Ionic Compounds
Ca2+ + 2 F- f
CaF2
Mg2+ + NO3- f
Mg(NO3)2
magnesium nitrate
Fe2+ + PO43- f
calcium fluoride
Fe3(PO4)2
iron(II) phosphate
Be sure to practice naming compounds.
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Properties of Ionic Compounds
Forming NaCl from Na and Cl2
• A metal atom can
transfer an
electron to a
nonmetal.
• The resulting
cation and anion
are attracted to
each other by
electrostatic
forces.
© 2009 Brooks/Cole - Cengage
PLAY MOVIE
PLAY MOVIE
66
Electrostatic Forces
PLAY MOVIE
The oppositely charged ions in ionic compounds are
attracted to one another by ELECTROSTATIC
FORCES.
These forces are governed by COULOMB’S LAW.
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Electrostatic Forces
COULOMB’S LAW
As ion charge increases, the attractive force
_______________.
As the distance between ions increases, the
attractive force ________________.
This idea is important and will come up
many times in future discussions!
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68
Electrostatic Forces
COULOMB’S LAW
See Active Figure 2.21
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70
Importance of Coulomb’s Law
NaCl, Na+ and Cl-,
m.p. 804 oC
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MgO, Mg2+ and O2m.p. 2800 oC
Molecular Compounds
Compounds without Ions
CO2 Carbon dioxide
PLAY MOVIE
CH4 methane
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BCl3 boron trichloride
71
Naming Molecular
Compounds
CO2 Carbon dioxide
PLAY MOVIE
CH4 methane
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BCl3
boron trichloride
All are
formed from
two or more
nonmetals.
Ionic
compounds
generally
involve a metal
and nonmetal
(NaCl)
72
Counting Atoms
Mg burns in air (O2) to
produce white magnesium
oxide, MgO.
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
How can we figure out how
much oxide is produced
from a given mass of Mg?
73
74
Counting Atoms
Chemistry is a quantitative
science—we need a
“counting unit.”
MOLE
1 mole is the amount of
substance that contains as
many particles (atoms,
molecules) as there are in
12.0 g of 12C.
© 2009 Brooks/Cole - Cengage
518 g of Pb, 2.50 mol
Particles in a Mole
Avogadro’s Number
Amedeo Avogadro
1776-1856
6.0221415 x
23
10
There is Avogadro’s number of
particles in a mole of any substance.
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75
Molar Mass
1 mol of 12C
= 12.00 g of C
= 6.022 x 1023 atoms
of C
12.00 g of 12C is its
MOLAR MASS
Taking into account all
of the isotopes of C,
the molar mass of C is
12.011 g/mol
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77
One-mole Amounts
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78
PROBLEM: What amount of Mg
is represented by 0.200 g? How
many atoms?
Mg has a molar mass of 24.3050 g/mol.
 1 mol 
-3
0.200 g 
=
8.23
x
10
mol

 24.31 g 
How many atoms in this piece of Mg?
8.23 x 10
-3
 6.022 x 1023 atoms 
mol 

1
mol


= 4.95 x 1021 atoms Mg
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MOLECULAR WEIGHT
AND MOLAR MASS
Molecular weight = sum of the
atomic weights of all atoms in the
molecule.
Molar mass = molecular weight in
grams
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79
What is the molar
mass of ethanol,
C2H6O?
1 mol contains
2 mol C (12.01 g C/1 mol) = 24.02 g C
6 mol H (1.01 g H/1 mol) = 6.06 g H
1 mol O (16.00 g O/1 mol) = 16.00 g O
TOTAL = molar mass = 46.08 g/mol
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81
Tylenol
• Formula =
C8H9NO2
• Molar mass =
© 2009 Brooks/Cole - Cengage
151.2 g/mol
Molar Mass
Note that the mass of water is
included in the molar mass of a
compound.
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82
Empirical & Molecular
Formulas
A pure compound always consists of the
same elements combined in the same
proportions by weight.
Therefore, we can express molecular
composition as PERCENT BY
WEIGHT
Ethanol, C2H6O
52.13% C
13.15% H
34.72% O
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83
Percent Composition
Consider some of the family of nitrogenoxygen compounds:
NO2, nitrogen dioxide and closely
related, NO, nitrogen monoxide (or
nitric oxide)
Chemistry of NO,
nitrogen monoxide
Structure of NO2
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
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85
Percent Composition
Consider NO2, Molar mass = ?
What is the weight percent of N and of
O?
Wt. % N =
Wt. % O =
14.0 g N
x 100% = 30.4 %
46.0 g NO2
2(16.0 g O per mol NO2 )
46.0 g NO2
x 100% = 69.6%
What are the weight percentages of
N and O in NO?
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How to Determine a Formula?
Mass spectrometer
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86
Mass Spectrum of Ethanol
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87
88
Determining Formulas
In
chemical analysis we determine
the % by weight of each element in a given
amount of pure compound and derive the
EMPIRICAL or SIMPLEST formula.
PROBLEM:
A compound of B and H
is 81.10% B. What is its empirical
formula?
© 2009 Brooks/Cole - Cengage
A compound of B and H is 81.10% B. What is
its empirical formula?
• Because it contains only B and H, it
must contain 18.90% H.
• In 100.0 g of the compound there are
81.10 g of B and 18.90 g of H.
• Calculate the number of moles of each
constituent.
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89
A compound of B and H is 81.10% B. What is
its empirical formula?
Calculate the number of moles of each
element in 100.0 g of sample.
 1 mol 
81.10 g B 
= 7.502 mol B

 10.81 g 
 1 mol 
18.90 g H 
= 18.75 mol H

 1.008 g 
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90
A compound of B and H is 81.10% B. What is
its empirical formula?
Now, recognize that atoms combine in the
ratio of small whole numbers.
1 atom B + 3 atoms H f 1 molecule BH3
or
1 mol B atoms + 3 mol H atoms f
1 mol BH3 molecules
Find the ratio of moles of elements in
the compound.
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91
A compound of B and H is 81.10% B. What is
its empirical formula?
Take the ratio of moles of B and H. Always
divide by the smaller number.
18.75 mol H
2.499 mol H
2.5 mol H
=
=
7.502 mol B
1.000 mol B
1.0 mol B
But we need a whole number ratio.
2.5 mol H/1.0 mol B = 5 mol H to 2 mol B
EMPIRICAL FORMULA = B2H5
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92
A compound of B and H is 81.10% B.
Its empirical formula is B2H5. What is
its molecular formula?
Is the molecular formula B2H5, B4H10,
B6H15, B8H20, etc.?
B 2H 6
B2H6 is one example of this class of compounds.
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93
A compound of B and H is 81.10% B. Its empirical
formula is B2H5. What is its molecular formula?
We need to do an EXPERIMENT to find
the MOLAR MASS.
Here experiment gives 53.3 g/mol
Compare with the mass of B2H5
= 26.66
g/unit
Find the ratio of these masses.
2 units of B2H5
53.3 g/mol
=
26.66 g/unit of B2H5
1 mol
Molecular formula = B4H10
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94
95
DETERMINE THE FORMULA OF A
COMPOUND OF Sn AND I
Sn(s) + some I2(s) f SnIx
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96
Data to Determine the formula of
a Sn—I Compound
• Reaction of Sn and I2 is done using
excess Sn.
• Mass of Sn in the beginning = 1.056 g
• Mass of iodine (I2) used = 1.947 g
• Mass of Sn remaining = 0.601 g
• See p. 93
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97
Tin and Iodine Compound
Find the mass of Sn that combined with
1.947 g I2.
Mass of Sn initially =
1.056 g
Mass of Sn recovered = 0.601 g
Mass of Sn used =
0.455 g
Find moles of Sn used:
 1 mol 
-3
0.455 g Sn 
=
3.83
x
10
mol Sn

 118.7 g 
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98
Tin and Iodine Compound
Now find the number of moles of I2 that
combined with 3.83 x 10-3 mol Sn. Mass
of I2 used was 1.947 g.
 1 mol I2 
-3
1.947 g I2 
=
7.671
x
10
mol I2

 253.81 g I2 
How many mol of iodine atoms?
-3
7.671 x 10
 2 mol I atoms 
mol I2 

 1 mol I2 
= 1.534 x 10-2 mol I atoms
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99
Tin and Iodine Compound
Now find the ratio of number of moles of moles
of I and Sn that combined.
1.534 x 10-2 mol I
3.83 x 10-3
4.01 mol I
=
1.00 mol Sn
mol Sn
Empirical formula is
© 2009 Brooks/Cole - Cengage
SnI4