CHAPTER 2
Chemical Formulas and Composition
Stoichiometry
Chapter Goals
1. Chemical Formulas
2. Ions and Ionic Compounds
3. Names and Formulas of Some Ionic Compounds
4. Atomic Weights
5. The Mole
6. Formula Weights, Molecular Weights, and Moles
7. Percent Composition and Formulas of Compounds
8. Derivation of Formulas from Elemental Composition
9. Determination of Molecular Formulas
10. Some Other Interpretations of Chemical Formulas
11. Purity of Samples
2
3
Chemical Formulas
• Chemical formula shows the chemical composition of the substance.– ratio of the elements present in the molecule or
compound
• He, Au, Na – monatomic elements• O2, H2, Cl2 – diatomic elements• O3, P4, S8 - more complex elements• H2O, C12H22O11 – compounds
Substance consists of two or more elements
4
Chemical Formulas
Compound 1 Molecule ContainsHCl 1 H atom & 1 Cl atom
H2O 2 H atoms & 1 O atom
NH3 1 N atom & 3 H atoms
C3H8 3 C atoms & 8 H atoms
5
Ions and Ionic Compounds
• Ions are atoms or groups of atoms that possess an electric charge.
• Two basic types of ions:– Positive ions or cations
• one or more electrons less than neutral• Na+, Ca2+, Al3+
• NH4+ - polyatomic cation
– Negative ions or anions• one or more electrons more than neutral• F-, O2-, N3-
• SO42-, PO4
3- - polyatomic anions
6
Ions and Ionic Compounds
• Sodium chloride– table salt is an ionic compound
Systematic Naming
• There are too many compounds to remember the names of them all.
• Compound is made of two or more elements.
• Name should tell us how many and what type of atoms.
Charges on ions
• For most of Group A elements, location on the Periodic Table can tell what kind of ion they form
• Elements in the same group have similar properties.
• Including the charge when they are ions.
2+
1+
3+ 3- 2- 1-
Charge in groups 1A, 2A and 3A is the group number
Naming Monatomic Ions Naming Monatomic Ions
Name these
Na1+ Sodium ionCa2+ Calcium ionAl3+ Aluminum ion
Li1+ Lithium ion
Write Formulas for these
Potassium ion K1+
Magnesium ion Mg2+
Copper(II) ion Cu2+
Chromium(VI) ion Cr6+
Barium ion Ba2+
Mercury(II) ion Hg2+
Name these
Cl1- Chloride ion N3- Nitride ion Br1- Bromide ion O2- Oxide ion Ga3+ Gallium ion
Write these
Sulfide ion S2-
Iodide ion I1-
Phosphide ion P3-
Strontium ion Sr2+
Binary Ionic Compounds Binary Ionic Compounds
• Binary Compounds – Compounds composed of two different elements
– Binary ionic cmpd, total # of positive and negative charges must equal
Writing Formulas for Binary Ionic Writing Formulas for Binary Ionic Compounds Compounds
• Write the symbols for the ions side by side. ALWAYS write the cation first!
Al3+ O2-
• Cross over the charges by using the absolute value of each ion’s charge as the subscript for the other ion
Al23+
O32-
• Check that the subscripts are in smallest whole number ratio
Al2O3
Naming Binary Ionic CompoundsNaming Binary Ionic Compounds
Al2O3
– 1. Name the cation(don’t change) Aluminum
– 2. Name the anion (drop add-ide) Oxide
Aluminum Oxide
The Stock System of Nomenclature The Stock System of Nomenclature
• Most transition metals can have more than one type of charge.
• Indicate the charge with Roman numerals in parenthesis.– Fe2+Iron(II) – Fe3+ Iron(III)
• Roman numerals are never used: – For anions – For metals that form
only one ion
Co2+
Cobalt(II) ion
CuCl2
Copper(II) chloride
Naming ions Cont…..
• A few, like silver, zinc and cadmium only form one kind of ion
• Don’t get roman numerals
• Ag+ silver ion
• Zn2+ zinc ion
• Cd2+ cadmium ion
Compounds Containing Polyatomic Compounds Containing Polyatomic Ions Ions
• Naming a series of similar polyatomic ions – NO2
- NO3-
– Nitrite Nitrate– Most common ion is given –ate ending, ion
with one less oxygen -ite
• Naming compounds containing polyatomic ions – Same as for monatomic ions
Naming Binary Molecular Naming Binary Molecular Compounds Compounds
A. Binary Molecular Compounds – 1. Covalently bonded molecules containing only two elements,
both nonmetals B. Naming
– 1. Least electronegative element is named first • Measure of the ability of an atom in chemical compound to
attract e-• Tends to increase across periods decrease or remain the
same down– 2. First element gets a prefix if there is more than 1 atom of that
element – 3. Second element ALWAYS gets a prefix, and an “-ide” ending
• Examples: N2O3 = dinitrogen trioxide
• CO = carbon monoxide, not monocarbon monoxide
22
Names and Formulas of Some Ionic Compounds
• Table 2-1 gives names of several molecular compounds.– You must know all of the molecular
compounds from Table 2-2.
• Some examples are:– H2SO4 - sulfuric acid– FeBr2 - iron(II) bromide– C2H5OH - ethanol
23
Names and Formulas of Some Ionic Compounds
• Table 2-2 displays the formulas, charges, and names of some common ions– You must know the names, formulas, and
charges of the common ions in table 2-3.
• Some examples are:– Anions - Cl1-, OH1-, SO4
2-, PO43-
– Cations - Na1+, NH41+, Ca2+, Al3+
Anion Ending Acid Name
-ide hydro-(stem)-ic acid
-ate (stem)-ic acid
-ite (stem)-ous acid
Acid Nomenclature
C. Johannesson
Acid Nomenclature
h yd ro - p re fix-ic e n d ing
2 e le m e n ts
-a te e nd ingb e com es-ic e nd ing
-ite e nd ingb e com es
-o us e nd ing
n o h yd ro - p re fix
3 e le m e n ts
AC ID Ss ta rt w ith 'H '
HBr
H2CO3
H2SO3
2 elements, -ide
3 elements, -ate
3 elements, -ite
hydrobromic acid
carbonic acid
sulfurous acid
Acid Nomenclature
hydrofluoric acid
sulfuric acid
nitrous acid
2 elements
3 elements, -ic
3 elements, -ous
HF
H2SO4
HNO2
Acid Nomenclature
H+ F-
H+ SO42-
H+ NO2-
28
Names and Formulas of Some Ionic Compounds
You do it!
• What is the formula of nitric acid?
• HNO3
• What is the formula of sulfur trioxide?
• SO3
• What is the name of FeBr3?
• iron(III) bromide
29
Names and Formulas of Some Ionic Compounds
You do it!
• What is the name of K2SO3?
• potassium sulfite
• What is charge on sulfite ion?
• SO32- is sulfite ion
• What is the formula of ammonium sulfide?
• (NH4)2S
30
Names and Formulas of Some Ionic Compounds
You do it!
• What is the charge on ammonium ion?
• NH41+
• What is the formula of aluminum sulfate?
• Al2(SO4)3
• What are the charges on both ions?
• Al3+ and SO42-
31
32
Atomic Weights
• Weighted average of the masses of the constituent isotopes of an element.– Tells us the atomic masses of
every known element.– Lower number on periodic
table.
33
The Mole
• A number of atoms, ions, or molecules that is large enough to see and handle.
• A mole = number of things– Just like a dozen = 12 things– One mole = 6.022 x 1023 things
• Avogadro’s number = 6.022 x 1023 – Symbol for Avogadro’s number is NA.
1 mole of hockey pucks would equal the mass of the moon!
• 1 mole of pennies would cover the Earth 1/4 mile deep!
1 mole of basketballs would fill a bag the size of the earth!
35
The Mole
Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.
36
The Mole
Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.
Mg g 104.04atoms Mg mol 1
Mgg24.30
atoms Mg 106.022
atoms Mg mol 1atom Mg 1Mg g ?
23
23
37
The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.
38
The Mole
Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.
atoms Mg102.48atoms Mgmol 1
atoms Mg106.022
Mgg 24.30
Mg mol 1 Mgg 101.00atoms Mg?
1623
6
39
The Mole
Example 2-3: How many atoms are contained in 1.67 moles of Mg?
40
The Mole
Example 2-3. How many atoms are contained in 1.67 moles of Mg?
atoms Mg101.00
Mgmol 1
atoms Mg106.022Mg mol 1.67atoms Mg?
24
23
41
The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?
You do it!You do it!
42
The Mole
Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?
Mg mol 02.3
Mg g 24.30
atoms Mg mol 1 Mg g 4.73Mg mol ?
IT IS IMPERATIVE THAT YOU KNOWHOW TO DO THESE PROBLEMS
43
Formula Weights, Molecular Weights, and Moles
• How do we calculate the formula weight of a compound?– sum the atomic weight of each atom
The formula weight of propane, C3H8, is:
3 C 3 12.01 amu 36.03 amu
8 H 8 1.01 amu 8.08 amu
Forula Weight 44.11 amu
44
Formula Weights, Molecular Weights, and Moles
The formula weight of calcium nitrate, Ca(NO3)2, is:
You do it!You do it!
45
Formula Weights, Molecular Weights, and Moles
amu 164.10 massMolar
amu 96.00 amu 16.006 O6
amu 28.02 amu 14.012 N2
amu 40.08 amu 40.081 Ca1
46
Formula Weights, Molecular Weights, and Moles
• One Mole of Contains– Cl2 or 70.90g 6.022 x 1023 Cl2 molecules
2(6.022 x 1023 ) Cl atoms
47
Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.
48
Formula Weights, Molecular Weights, and Moles
Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.
molecules 10 02.1
HC mole 1
molecules HC 106.022
HC g 44.11
HC mole 1
HC g 74.6molecules HC ?
24
83
8323
83
83
8383
49
Formula Weights, Molecular Weights, and Moles
Example 2-6: What is the mass of 10.0 billion propane molecules?
You do it!You do it!
50
Formula Weights, Molecular Weights, and Moles
Example 2-6. What is the mass of 10.0 billion propane molecules?
8313
83
83
238310
83
HC of g 1032.7HC mole 1
HC g 11.44
molecules 106.022
HC mole 1molecules 10 1.00 molecules HC g ?
51
Formula Weights, Molecular Weights, and Moles
Example 2-7: How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
You do it!You do it!
52
Formula Weights, Molecular Weights, and Moles
Example 2-7a. How many moles are contained in 60.0 g of ozone, O3
moles 25.1O g 0.48
mole 1O g 0.60O moles ?
333
53
Formula Weights, Molecular Weights, and Moles
Example 2-7a: How many moles are contained in 60.0 g of ozone, O3?
54
Formula Weights, Molecular Weights, and Moles
Example 2-7a. How many moles are contained in 60.0 g of ozone, O3
moles 25.1O g 0.48
mole 1O g 0.60O moles ?
333
55
Formula Weights, Molecular Weights, and Moles
Example 2-7b: How many molecules are contained in 60.0 g of ozone, O3?
56
Formula Weights, Molecular Weights, and Moles
Example 2-7b. How many molecules are contained in 60.0 g of ozone, O3
323
23
3
O molecules 107.53
mole 1
molecules 106.022moles 25.1O molecules ?
57
Formula Weights, Molecular Weights, and Moles
Example 2-7c: How many oxygen atoms are contained in 60.0 g of ozone, O3?
58
Formula Weights, Molecular Weights, and Moles
Example 2-7c. How many oxygen atoms are contained in 60.0 g of ozone, O3
O atoms 1026.2
molecule O 1
atoms O 3O molecules 107.53atoms O ?
24
33
23
59
Formula Weights, Molecular Weights, and Moles
Example 2-8: Calculate the number of O atoms in 26.5 g of Li2CO3.
You do it!You do it!
60
Formula Weights, Molecular Weights, and Moles
Example 2-8. Calculate the number of O atoms in 26.5 g of Li2CO3.
atoms O 106.49
COLi unit formula 1
atoms O 3
COLi mol 1
COLi unitsform.106.022
COLi g 73.8
COLi mol 1COLi g 26.5atoms O ?
23
3232
3223
32
3232
61
Formula Weights, Molecular Weights, and Moles
• Occasionally, we will use millimoles. – Symbol - mmol– 1000 mmol = 1 mol
• For example: oxalic acid (COOH)2
– 1 mol = 90.04 g – 1 mmol = 0.09004 g or 90.04 mg
62
Formula Weights, Molecular Weights, and Moles
Example 2-9: Calculate the number of mmol in 0.234 g of oxalic acid, (COOH)2.
You do it!You do it!
63
Formula Weights, Molecular Weights, and Moles
Example 2-9: Calculate the number of mmol in 0.234 g of oxalic acid, (COOH)2.
22
2
22
(COOH) mmol 2.60(COOH) g 0.09004
(COOH) mmol 1
(COOH) g 234.0(COOH) mmol ?
64
65
Percent Composition and Formulas of Compounds
• % composition = mass of an individual element in a compound divided by the total mass of the compound x 100%
Determine the percent composition of C in C3H8.
66
Percent Composition and Formulas of Compounds
What is the percent composition of H in C3H8?
You do it!You do it!
67
Percent Composition and Formulas of Compounds
• % composition = mass of an individual element in a compound divided by the total mass of the compound x 100%
Determine the percent composition of C in C3H8.
81.68%
100%g 44.11
g 12.013
100%HC mass
C mass C %
83
68
Percent Composition and Formulas of Compounds
What is the percent composition of H in C3H8?
81.68%100%18.32%
or
%18.32100%g 44.11
g 1.018
100%HC
H8
100%HC mass
H massH %
83
83
69
Percent Composition and Formulas of Compounds
Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures.
You do it!
70
Percent Composition and Formulas of Compounds
Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures.
100% Total
O 48.0% 100%g 399.9
g 16.012 100%
)(SOFe
O12 O %
S 24.1% 100%g 399.9
g 32.13 100%
)(SOFe
S3 S %
Fe 27.9% 100%g 399.9
g 55.82 100%
)(SOFe
Fe2Fe %
342
342
342
71
Derivation of Formulas from Elemental Composition
• Empirical Formula - smallest whole-number ratio of atoms present in a compound– CH2 is the empirical formula for alkenes– No alkene exists that has 1 C and 2 H’s
• Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound– Ethene – C2H4
– Pentene – C5H10
• We determine the empirical and molecular formulas of a compound from the percent composition of the compound.– percent composition is determined experimentally
Empirical Formula
1. Find mass (or %) of each element.
2. Find moles of each element.
3. Divide moles by the smallest # to find subscripts.
4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.
73
Derivation of Formulas from Elemental Composition
Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?
74
Derivation of Formulas from Elemental Composition
Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?Make the simplifying assumption that we have 100.0 g of compound.
O mol 2.531O g 16.00
O mol1O g 40.50 O mol ?
Mn mol 0.6327Mn g 54.94
Mn mol 1Mn g 34.76 Mn mol ?
K mol 0.6327K g 39.10
K mol 1K g 24.74 K mol ?
75
Derivation of Formulas from Elemental Composition
Example 2-11
4KMnO is formula chemical thethus
O 40.6327
2.531Ofor
Mn 10.6327
0.6327Mnfor K 1
0.6327
0.6327Kfor
rationumber wholesmallest obtain
O mol 2.531O g 16.00
O mol1O g 40.50 O mol ?
Mn mol 0.6327Mn g 54.94
Mn mol 1Mn g 34.76 Mn mol ?
K mol 0.6327K g 39.10
K mol 1K g 24.74 K mol ?
76
Derivation of Formulas from Elemental Composition
Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound?
You do it!You do it!
77
Derivation of Formulas from Elemental Composition
Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound?
43OCo
:is formula scompound' theThus
O 43O 1.333 Co 33Co 1
number wholetofraction turn to 3 by both multipy
O1.3330.1110
0.1480O for Co 1
0.1110
0.1110Co for
ratio number wholesmallest find
O mol 0.1480O g 16.00
O mol1O g 2.368O mol ?
Co mol 0.1110Cog 58.93
Co mol 1Co g 6.541Co mol ?
Molecular FormulaMolecular Formula
1. Find the empirical formula.2. Find the empirical formula mass.3. Divide the molecular mass by the
empirical mass.4. Multiply each subscript by the
answer from step 3.
nmass EF
mass MF nEF
79
Determination of Molecular Formulas
Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?
80
Determination of Molecular Formulas
Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?
84HC
:is formula theThus
H mol 8H g 1.01
H mol 1H of g 8.10
C mol 4C g 12.0
C mol 1C of g 48.0
moles tomassesconvert
H of g 8.100.1437g 56.1
C of g 48.00.8563g 56.1
H is 14.37% and C is 85.63%
g 56.1 contains mol 1
Law of Multiple Proportions
• It is possible for two elements, A and B, to combine to form more than one compound.
• The ratios of the masses of element B that combine with a given mass of element A in each compound can be expressed by small whole numbers.
81
Law of Multiple Proportions
Example 2-14: Show that the compounds NO2 and N2O5 obey the law of multiple proportions.
82
Law of Multiple Proportions
Example 2-14: Show that the compounds NO2 and N2O5 obey the law of multiple proportions.
83
2
2 5
For NO
22
1
For N O
52.5
2
O atoms
N atoms
O atoms
N atoms
Some Other Interpretations of Chemical Formulas
Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH4)3PO4?
84
Some Other Interpretations of Chemical Formulas
Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH4)3PO4?
85
4 3 44 3 4
4 3 4
4 3 4
1 ( )? 45.3 ( )
149.09 ( )
1 30.9738
1 ( ) 1
9.41
mole NH POgrams P g NH PO
g NH PO
mole P g P
mole NH PO mole P
g P
86
Some Other Interpretations of Chemical Formulas
Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N?
87
Some Other Interpretations of Chemical Formulas
Example 2-16: What mass of ammonium phosphate, (NH4)3PO4, would contain 15.0 g of N?
434434
434434
434434
434
PO)(NH g 53.2PO)(NH mol 1
PO)(NH g 149.0PO)(NH mol 0.357
PO)(NH mol 0.357N mol 3
PO)(NH mol 1N mol 1.07
N mol 1.07N g 14.0
N mol 1N of g 15.0N mol ?
g/mol 149.0PO)(NH of massmolar
88
Purity of Samples
• The percent purity of a sample of a substance is always represented as
mass of pure substance% purity = 100%
mass of sample
- mass of sample includes impurities
89
Purity of Samples
Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in 250.0 g of this sample of Na3PO4?
90
Purity of Samples
Example 2-18: A bottle of sodium phosphate, Na3PO4, is 98.3% pure Na3PO4. What are the masses of Na3PO4 and impurities in 250.0 g of this sample of Na3PO4?
impurities g 4 =
PONa g 246 - sample g 250.0 = impurities g
PONa g 246=
sample g 100.0
PONa g 98.3sample g PONa g
sample g 100.0
PONa g 98.3factor unit
43
43
43
43
43
?
0.250?
91
Synthesis Problem
In 1986, Bednorz and Muller succeeded in making the first of a series of chemical compounds that were superconducting at relatively high temperatures. This first compound was La2CuO4 which superconducts at 35K. In their initial experiments, Bednorz and Muller made only a few mg of this material. How many La atoms are present in 3.56 mg of La2CuO4?
92
Synthesis Problem
How many La atoms are present in 3.56 mg of La2CuO4?
93
Synthesis Problem
How many La atoms are present in 3.56 mg of La2CuO4?
atoms La 1006.1CuOLa molecule
atoms La 2
CuOLa mol 1
CuOLa molecules 10022.6)CuOLa mol 1078.8(
CuOLa mol 1078.8CuOLa g 405.3
CuOLa mol 1
mg 1000
g 1CuOLa mg 3.56
g/mol 405.3=CuOLa of massmolar
19
42
42
4223
426
426
42
42
42
42
94
Group Activity
• Within a year after Bednorz and Muller’s initial discovery of high temperature superconductors, Wu and Chu had discovered a new compound, YBa2Cu3O7, that began to superconduct at 100 K. If we wished to make 1.00 pound of YBa2Cu3O7, how many grams of yttrium must we buy?