Molecules and Compounds

1
Molecules and Compounds
CHAPTER 3

• Read Chapter 3.
• Study all examples and complete all exercises.
• Complete all bold numbered problems.

2
Chapter 3 Outline

• Molecular Formula
• Molar Mass
• Empirical and Molecular Formula
• Nomenclature

3
Compounds Molecules
Buckyball, C60
NaCl, salt
4
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.

5
MOLECULAR FORMULAS

• Formula for glycine is C2H5NO2
• In one molecule there are
• 2 C atoms
• 5 H atoms
• 1 N atom
• 2 O atoms

6
WRITING FORMULAS

• Formula
• HOCH2CH2OH to show atom ordering
• or in the form of a structural formula

7
Molecular Modeling
8
Molecular Modeling
Drawing of glycine
Ball stick
Space-filling
9
Resources for Molecular Modeling

• Oxford Molecular/CAChe Scientific software on
  Saunders General Chemistry CD-ROM
• Rasmol and Chime on the Internet
• See http//www.saundercollege.com

10
ELEMENTS THAT EXIST AS MOLECULES
Allotrope of C
Buckyball, C60
11
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12
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.

13
Formation of Cations Anions
14
Formation of Cations Anions
F e- -- F-
Mg -- Mg2 2 e-
A cation forms when an atom loses one or more
electrons.
An anion forms when an atom gains one or more
electrons
15
PREDICTING ION CHARGES
MONATOMIC IONS

• In general
• metals (Mg) lose electrons --- cations
• nonmetals (F) gain electrons --- anions
• Li1 F1- LiF

16
Figure 3.7
Also
Sn4
Pb4
17
METALS

• M --- n e- Mn
• where n periodic group
• Na
• Mg2
• Al3
• Transition metals -- M2 or M3
• are most common

18
NONMETALS

• NONMETAL n e- ------ Xn- where n 8 -
  Group number
• C4- carbide
• N3- nitride
• O2- oxide
• F- fluoride

Bromine
19
POLYATOMIC IONS

• Groups of atoms with a charge.
• 
  (See back of periodic chart)

20
Some Common Polyatomic Ions
NO3- nitrate ion
HNO3 nitric acid
21
Some Common Polyatomic Ions

• NH4
• ammonium ion
• One of the few common polyatomic cations

22
Some Common Polyatomic Ions

• CO32-
• carbonate ion
• HCO3-
• bicarbonate ion
• hydrogen carbonate

23
Some Common Polyatomic Ions

• PO43-
• phosphate ion
• CH3CO2-
• acetate ion

24
Some Common Polyatomic Ions

• SO42-
• Sulfate ion
• SO32-
• Sulfite ion

25
Some Common Polyatomic Ions

• NO3-
• Nitrate ion
• NO2-
• Nitrite ion

26
COMPOUNDS FORMED FROM IONS

• CATION ANION COMPOUND
• Na Cl- NaCl
• A neutral compound requires equal number of
  and - charges.

27
IONIC COMPOUNDS
ammonium chloride, NH4Cl
NH4
Cl-
28
Some Ionic Compounds

• Ca2 2 F- CaF2
• Calcium fluoride
• Mg2 2 NO3- Mg(NO3)2
• Magnesium nitrate
• 3 Fe2 2 PO43- Fe3(PO4)2
• Iron(II) phosphate

Calcium Fluoride
29
Sample Questions

• Predict the charges for the ions formed from
• Se Ga
• P Sr
• Give the formula for each ion in Al2(SO4)3
• Give the formula for the ionic compound that
  forms between
• Na and S
• Ga and O
• Ba and N

Answers
30
Properties of Ionic CompoundsForming 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.

30
31
Electrostatic Forces

• The oppositely charged ions in ionic compounds
  are attracted to one another by ELECTROSTATIC
  FORCES.
• These forces are governed by
• COULOMBS LAW.

32
Electrostatic Forces

• COULOMBS 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!

(charge on)(charge on-)


Force of attraction
(distance between ions)2
33
Importance of Coulombs Law
NaCl, Na and Cl-, m.p. 801 oC
MgO, Mg2 and O2- m.p. 2800 oC
AlN, Al3 and N3- m.p. 2900 oC
34
Names of Compounds

• Rules for nomenclature are found in section 3.5
• STUDY them carefully!!
• We will be studying nomenclature in the
  laboratory in Experiment MA.

35
Counting Atoms

• Mg burns in air (O2) to produce white magnesium
  oxide, MgO.
• How can we figure out how much oxide is produced
  from a given mass of Mg?

36
Counting Atoms

• Chemistry is a quantitative science we need a
  counting unit.
• The MOLE
• 1 mole is the amount of substance that contains
  as many particles (atoms, molecules) as there are
  in 12.0 g of 12C.

37
Particles in a Mole
Avogadros Number Amedeo Avogadro 1776-1856

• 6.02 x 1023

There is Avogadros number of particles in a mole
of any substance.
38
Mole in Chemistry is NOT

• An informer / spy
• Dark spot on Cindy Crawfords upper lip
• Rodent that burrows in the ground
• A tunneling machine
• Wave break
• Spicy Mexican sauce

39
A mole is a convenient measuring tool.
Bakers Dozen
40
Mole mol 6.022 ? 1023 particles
41
Just as  1 doz eggs 12 eggs or   1 mol of eggs
6.022 ? 1023 eggs A mole is a Number 1 mol
of H 6.022 ? 1023 atoms of H 1 mol of O
6.022 ? 1023 atoms of O 1 mol of Al 6.022 ?
1023 atoms of Al 1 mol of Cr 6.022 ? 1023
atoms of Cr
42
In chemistry, the mol is more then just a number
1 mol ? amu 1.00 g
43
1 mol ? amu 1.00 g
Proof 1 mol 6.022 ? 1023 amu 1.661 ? 10-24
g   1 mol amu (6.022 ? 1023) ? (1.661 ? 10-24g)
1.00 g
44
Molar Mass

• 1 mol of 12C 12.00 g of C 6.02 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

45
Molar Mass

• 1 mol of 12C 12.00 g of C 6.02 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

Find molar mass from periodic table
46
PROBLEM How many moles are represented by 0.200
g of Mg?
0.200 g
1 mole 24.3 g

0.00823 mole

• How many atoms in this piece of Mg?

0.00823 mole
6.02 x 1023 atom 1 mole

4.95 x 1021 atom
47
MOLECULAR WEIGHT AND MOLAR MASS

• Molecular weight is the sum of the atomic
  weights of all atoms in the molecule.
• Molar mass molecular weight in grams

48
What is the molar mass of ethanol, C2H6O?

• 1 mol contains
• 2 mol C (12.0 g C/1 mol) 24.0 g C
• 6 mol H (1.0 g H/1 mol) 6.0 g H
• 1 mol O (16.0 g O/1 mol) 16.0 g O
• TOTAL molar mass 46.0 g/mol

49
Tylenol

• Formula C8H9NO2
• Molar mass 151.0 g/mol

50
How many moles of alcohol are there in a
standard can of beer if there are 21.3 g of
C2H6O?
21.3 g
1 mole 46.0 g

0.463 mole
51
How many molecules of alcohol are there in a
standard can of beer if there are 21.3 g of
C2H6O?
0.463 mole
6.02 x 1023 molecule 1 mole

2.79 x 1023 molecule
52
How many atoms of C are there in a standard can
of beer if there are 21.3 g of C2H6O?
2.79 x 1023 molecule
2 atom C 1 molecule

5.58 x 1023 atom C
Sample problems
53
Empirical and 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
• 52.2 C, 13 H,
• 34.8 O

54
Percent Composition

• Consider some of the family of nitrogen-oxygen
  compounds
• NO2, nitrogen dioxide and closely related, NO,
  nitrogen monoxide (or nitric oxide)

Structure of NO2
Chemistry of NO, nitrogen monoxide (nitric oxide)
55
Percent Composition

• Consider NO2, Molar mass ?
• What is the weight percent of N and of O?

(
16
.
0
g O per mole
)
2


O

x
1
00



69
.
6

Wt.

46
.
0
g
56
Percent Composition

• What are the weight percentages of N and O in NO?

16.0
g O


1
00




53.3

Wt.

O

30.0
g NO
57
Percent CompositionSamples Problems
1. Calculate the percent composition of H2O.

g O
16.0



Wt.

O


1
00


88.9

18.0 g H2O
58
Percent CompositionSamples Problems
2. Calculate the percent O in NaOH.
59
Percent CompositionSamples Problems
3. Calculate the percent O and the percent water
in CuSO4.5H2O.
g O

144.0

Wt.

O


1
00



57.69


249.6 g CuSO4 5H2O
60
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.

61
A compound of B and H is 81.10 B. What is its
empirical formula?
B H
1.00 mole B 2.5 mole H
2.00 mole B 5.0 mole H
B2H5
62
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.?

B2H6
B2H6 is one example of this class of compounds.
63
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.

Molecular formula B4H10
64
Determine the formula of a compound of Sn and I
using the following data.

• 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

65
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

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Tin and Iodine Compound
Sn I
1.00 mole Sn 4.01 mole I
SnI4
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More Problems

• 2. Calculate the formula for the iron sulfide
  that forms when 53.73g Fe react with 46.27 g of
  sulfur.

Fe S
1.00 1.50
2.00 3.00
Fe2S3
68
More Problems

• Calculate the empirical formula a compound
  containing 90.7 Pb and 9.33 O.

Pb O
1.00 1.33
3.00 3.99
Pb3O4
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More Problems

• Calculate the empirical formula for a compound
  containing 36.5 Na, 25.4 S and 38.1 O.

Na S O
1.59 0.791 2.38
0.791 0.791 0.791
2.01 1.00
3.01
Na2SO3
70
More Problems

• Calculate the empirical and molecular formulas
  for nicotine, 74.0 C, 8.7 H and 17.3 N, with a
  molar mass of 160 g/mole.

C H N
4.98 7.0
1.00
Empirical formula C5H7N
71
More Problems

• 7. Calculate the empirical and molecular formulas
  for nicotine, 74.0 C, 8.7 H and 17.3 N, with a
  molar mass of 160 g/mole.

Empirical formula C5H7N
Molecular formula C10H14N2
72
Practice Problems
Names/Formulas FeO Pb(C2H3O2)2 magnesium
bromide sodium chromate calcium phosphate
ammonium carbonate

73
Practice Problems

• (NH4)2S
• As2O3
• SO2
• silicon disulfide
• As2S5
• dinitrogen monoxide

74
Practice Problems

• As2S3
• dinitrogen pentoxide
• silicon tetrabromide
• diphosphorus pentoxide
• HBrO3
• H3PO4(aq)
• H2CO3

75
Practice Problems

• Calculations
• 1. 26 g H2 is how many moles H2?
• 2. 4.25 x 1021 molecules NH3 is how many grams
  NH3?
• 3. 1.5 x 102 formula units KClO3 is how many
  moles KClO3
• 4. 0.0042 mole Fe2O3 is how many formula units
  Fe2O3?
• 5. 2.15 moles MgSO4.7H2O is how many g MgSO4.7H2O?

76
Practice Problems

• 6. 25 molecules HBr is how many mole HBr?
• 7. .00002 g Sn is how many atoms Sn?
• 8. 7.25 mole H2S is how many g H2S?
• 9. 5.2 g Sr(OH)2is how many formula units
  Sr(OH)2?
• 10. How many moles of CCl4 will contain 2.4 g of
  chlorine?

77
Practice Problems

• 11. 19 g of HNO3 contains how many
• a) molecules of HNO3? b) grams of O?
• 12. Calculate the percent composition of NaCl.
• 13. Calculate the empirical and molecular
  formulas for a compound containing 43.7g P and
  56.3g O, with a molar mass of 140 g/mole.

78
Practice Problems Answers

• Iron(II) oxide, ferrous oxide
• lead(II) acetate, plumbous acetate
• MgBr2 Na2CrO4 Ca3(PO4)2
• (NH4)2CO3 ammonium sulfide
• diarsenic trioxide sulfur dioxide
• SiS2 As2S5 N2O
• diarsenic trisulfide N2O5 SiBr4
• P2O5 bromic acid phosphoric acid
• carbonic acid

79
Practice Problems Answers

• 1. 13 mole 2. 0.120 g
• 3. 2.5 x 10-22 mole 4. 2.5 x 1021 atom
• 5. 530. g 6. 4.2 x 10-23 mole
• 7. 1 x 1017 atom 8. 247 g
• 9. 2.6 x 1022 formula units
• 10. 0.017 mole
• 11. a) 1.8 x 1023 molecule
• b) 14 g
• 12. 39.3, 60.7 13. C5H7N
• 14. P2O5

80
Sample Questions

• Predict the charges for the ions formed from
• Se -2 Ga 3 Al3
• P -3 Sr 2 SO42-
• Give the formula for each ion in Al2(SO4)3
• Give the formula for the ionic compound that
  forms between
• Na and S Na2S
• Ga and O Ga2O3
• Ba and N Ba3N2

81
Sample Problems
1. 2.5 mole S ? atom S

2.5 mole
6.02 x 1023 atom 1 mole

1.5 x 1024 atom S
82
Sample Problems
2. 2.1 mole Zn ? g Zn
2.1 mole
65.4 g 1 mole

140 g Zn
83
Sample Problems
3. 1.42 g Mg ? atom Mg
1.42 g
1 mole 24.3 g
6.02 x 1023 atom 1 mole

3.55 x 1022 atom
84
Sample Problems
4. 125.2 g O2 ? mole O2
125.2 g
1 mole 32.0 g

3.91 mole O2
85
Sample Problems
1. 1.5 mole H2O ? g H2O
1.5 mole
18.0 g 1 mole

27 g H20
86
Sample Problems
2. 1.50 mole CCl4 ? molecules CCl4
1.5 mole
6.02 x 1023 molecule 1 mole

9.03 x 1023 molecule
87
Sample Problems
3. 1.25 mole CaCl2 ? formula units CaCl2
1.25 mole
6.02 x 1023 formula unit 1 mole

7.52 x 1023 formula unit
88
Sample Problems
4. 2.5 g K ? g KOH
2.5 g K
1 mole K 39.1 g K
1 mole KOH 1 mole K
56.1 g KOH 1 mole KOH

3.6 g KOH
89
Sample Problems
5. 34.5 g CaCO3 ? g O
34.5gCaCO3
1 moleCaCO3 100.1gCaCO3
3 moleO 1 moleCaCO3
16.0 gO 1 moleO

16.5 g O