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Chemical Formulas
and Composition
Stoichiometry
1
Chapter Goals
1. Chemical Formulas
2. Ions and Ionic Compounds
3. Names and Formulas of Some Ionic
Compounds
4. Atomic Weights
5. The Mole
2
Chapter Goals
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
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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
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Chemical Formulas
Compound 1 Molecule Contains
HCl
H2O
NH3
C3H8
1 H atom & 1 Cl atom
2 H atoms & 1 O atom
1 N atom & 3 H atoms
3 C atoms & 8 H atoms
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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-, PO43- - polyatomic anions
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Ions and Ionic Compounds
• Sodium chloride
– table salt is an ionic compound
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Names and Formulas of
Some Ionic Compounds
• Tables 1(p221) and 2 (p226) displays the
formulas, charges, and names of some
common ions
• Some examples are:
– Anions - Cl1-, OH1-, SO42-, PO43– Cations - Na1+, NH41+, Ca2+, Al3+
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Names and Formulas of
Some Ionic Compounds
• Formulas of ionic compounds are determined by
the charges of the ions.
– Charge on the cations must equal the charge on the
anions.
– The compound must be neutral.
•
•
•
•
NaCl
KOH
CaSO4
Al(OH)3
sodium chloride
(Na1+ & Cl1-)
potassium hydroxide(K1+ & OH1-)
calcium sulfate
(Ca2+ & SO42-)
aluminum hydroxide (Al3+ & 3 OH1-)
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• Types of ions:
• Simple positive and negative ions are
monoatomic in nature. We know their
charges from their position on the periodic
table.
• What about d block (transition metals)
elements that can lose a variable number
of electrons?
Chapter 7
• For example copper can lose one or two
electrons, producing :
• Cu+ copper I ion (copper one ion)
• Cu+2 copper II ion (copper two ion)
• So if we know the name of the ionic
compound we will know the charge on the
transition metal ion, likewise if we know the
formula of the ionic compound we will know
the charge on the ion (negative ions always
have the same charge)
Chapter 7
• All ionic compounds are electrically neutral-so the
+ charges must equal the negative.
• EXAMPLES OF THE CRISS-CROSS METHOD
• Poly atomic ions: It is possible for a group of
atoms to be bonded together and posses a
charge-thus the term polyatomic.
• YOU MUST MEMORIZE THE FOLLOWING
IONS!! Their name, formula and charge.
• Nitrate, Sulfate, Phosphate, Carbonate,
Bicarbonate, hydroxide , chlorate and
permanganate.
•
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
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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 SO4214
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.
• How do we know what the
values of these numbers
are?
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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.
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The Mole
Example 2-1: Calculate the mass of a single Mg
atom, in grams, to 3 significant figures.
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The Mole
Example 2-2: Calculate the number of atoms in
one-millionth of a gram of Mg to 3 significant
figures.
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The Mole
Example 2-3: How many atoms are contained in
1.67 moles of Mg?
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The Mole
Example 2-4: How many moles of Mg atoms are
present in 73.4 g of Mg?
You do it!
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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
8  H  8  1.01 amu
 36.03 amu
 8.08 amu
Forula Weight
 44.11 amu
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Formula Weights, Molecular
Weights, and Moles
The formula weight of calcium nitrate, Ca(NO3)2,
is:
You do it!
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Formula Weights, Molecular
Weights, and Moles
• One Mole of
– Cl2 or 70.90g
Contains
6.022 x 1023 Cl2 molecules
2(6.022 x 1023 ) Cl atoms
– C3H8
You do it!
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Formula Weights, Molecular
Weights, and Moles
Example 2-5: Calculate the number of C3H8
molecules in 74.6 g of propane.
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Formula Weights, Molecular
Weights, and Moles
Example 2-6: What is the mass of 10.0 billion
propane molecules?
You do it!
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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!
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Formula Weights, Molecular
Weights, and Moles
Example 2-7a: How many moles are contained in
60.0 g of ozone, O3?
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Formula Weights, Molecular
Weights, and Moles
Example 2-7b: How many molecules are contained
in 60.0 g of ozone, O3?
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Formula Weights, Molecular
Weights, and Moles
Example 2-7c: How many oxygen atoms are
contained in 60.0 g of ozone, O3?
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Formula Weights, Molecular
Weights, and Moles
Example 2-8: Calculate the number of O atoms in
26.5 g of Li2CO3.
You do it!
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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!
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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.
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Percent Composition and
Formulas of Compounds
What is the percent composition of H in C3H8?
You do it!
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Percent Composition and
Formulas of Compounds
Example 2-10: Calculate the percent composition
of Fe2(SO4)3 to 3 significant figures.
You do it!
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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
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Derivation of Formulas from
Elemental Composition
• We determine the empirical and molecular formulas of a
compound from the percent composition of the
compound.
– percent composition is determined experimentally
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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?
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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!
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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?
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