ATOMIC MASS The mass of an individual atom

1
CHAPTER 8 CHEMICAL COMPOSITION
ATOMIC MASS The mass of an individual atom
ATOMIC MASS UNIT (amu or u) One twelfth the
mass of a carbon-12 atom, equal to 1.66 x 10-24 g
Atomic masses are measured today with mass
spectrometers
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20Ne 19.99 u 90.92 21Ne 20.99 u 0.26 22Ne
21.99 u 8.82
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Any sample of neon contains the same percentage
of each isotope ELEMENTS ATOMIC MASS - Found on
the Periodic Table, the average atomic mass of
all of the elements naturally occurring isotopes
The atomic mass of neon is 20.18 u The atomic
mass of calcium is
40.08 u
4B-3
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Atomic masses allowed chemists to count atoms by
weighing them
Gummy Worms
Jordan Almonds
8 g each
4 g each
200 g
100 g
These 2 samples contain the same number of pieces
of candy
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1 Ne atom 20.18 u
1 Ca atom 40.08 amu
20.18 g Ne
40.08 g Ca
These two samples contain the same number of atoms
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The number of atoms contained in 20.18 g neon or
40.08 g calcium (numerically the elements atomic
mass, but measured in grams) is called a MOLE
For neon 20.18 g Ne 1 mol Ne atoms
For calcium 40.08 g Ca 1 mol Ca atoms

MOLAR MASS The mass of one mole of atoms of an
element
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Experimentally it has been shown that a mole is
6.022 ? 1023 atoms
AVOGADROS NUMBER 6.022 ? 1023
1 pair 2 shoes 1 dozen 12
eggs 1 mole 6.022 ? 1023 atoms The
number of atoms in 20.18 g Ne The number of
atoms in 40.08 g Ca
For shoes For eggs For atoms
MOLE The number of atoms contained in 12 g of
carbon-12
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The Elements Molar Mass The mass necessary to
have 1 mole of atoms of the element, measured in
grams 1 mole O atoms 15.9994 g
The Elements Atomic Mass The average atomic mass
of the elements isotopes, measured in amus 1
average O atom 15.9994 u
A Molar mass may be rounded to the hundredths
place as long as it doesnt limit the number of
significant figures in a calculation
4B-8
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Calculate the mass of a sample containing 2.50
moles of phosphorus atoms
For phosphorus 30.97 g P 1 mol P

x 30.97 g P _____________ 1 mol P
77.4 g P
2.50 mol P
Calculate the moles of atoms contained in a 10.0
g sample of potassium
4B-9
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Calculate the number of atoms contained in 4.50
moles of sodium
For any matter 6.022 x 1023 atoms 1
mol 2.50 mol Na
x 6.022 x 1023 atoms Na

_____________________________
1 mol Na
2.71 x 1024 atoms Na

Calculate the moles of atoms in a sample of 1.50
? 1023 tungsten atoms
4B-10
11
Calculate the number of atoms contained in a 1.00
g sample of iron
For iron 55.85 g Fe 1 mol Fe For any
matter 6.022 x 1023 atoms 1 mol 1.00 g Fe

x 6.022 x 1023 atoms Fe
_____________________________ 1
mol Fe
1.08 ? 1022 atoms Fe
x 1 mol Fe _______________ 55.85 g
Fe
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MOLAR MASSES OF COMPOUNDS
MOLAR MASS The mass of one mole of molecules of
a molecular substance, or one mole of formula
units of an ionic substance
Sodium chloride, NaCl 1 mole of NaCl contains 1
mole of Na and 1 mole of Cl- 1 mol Na (22.99
g/mol) 22.99 g 1 mol Cl- (35.45
g/mol) 35.45 g 58.44 g
The mass necessary to have 1 mole of NaCl formula
units (1 mole of Na ions and 1 mole of Cl- ions)
4B-12
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Sucrose, C12H22O11 1 mole of C12H22O11 contains
12 moles C, 22 moles H, and 11 moles of O 12 mol
C (12.01 g/mol) 144.12 g 22 mol H (1.01
g/mol) 22.22 g 11 mol O (16.00 g/mol) 176.00
g 342.34 g
The mass necessary to have 1 mole of C12H22O11
molecules (12 moles C atoms, 22 moles H atoms, 11
moles O atoms)
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Calcium hydroxide, Ca(OH)2 1 mol Ca (40.08
g/mol) 40.08 g 2 mol O (16.00 g/mol) 32.00
g 2 mol H (1.01 g/mol) 2.02 g 74.10 g
4B-14
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Calculate the mass of a sample containing 3.50
moles of magnesium fluoride. MgF2 1 mol Mg
(24.31 g/mol) 24.31 g 2 mol F (19.00
g/mol) 38.00 g 62.31 g
? 62.31 g MgF2 1 mol MgF2
x 62.31 g MgF2 __________________ 1 mol
MgF2
218 g MgF2
3.50 mol MgF2
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Calculate the number of moles of zinc bromide
present in a 10.0 g sample of the compound.
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PERCENT COMPOSITION OF COMPOUNDS BY MASS
Calculate the percent composition by mass of iron
(III) oxide.
Fe2O3 2 mol Fe (55.85 g/mol) 111.70 g 3 mol
O (16.00 g/mol) 48.00 g 159.70 g
111.70 g Fe ? 100 _________________________
_______ 159.70 g Fe2O3
Fe
69.944 Fe
48.00 g O ? 100 ________________________
________ 159.70 g Fe2O3
O
30.06 O
4B-17
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Calculate the percent composition by mass of
aluminum sulfate.
Al2(SO4)3 2 mol Al (26.98 g/mol) 53.96 g
3 mol S (32.07 g/mol) 96.21 g 12 mol O (16.00
g/mol) 192.00 g 342.17 g
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A sample of a compound containing Mn and O had a
mass of 2.59 g. The compound was heated to drive
off the O, and the final residue had a mass of
1.63g. Find the percent composition by mass of
the compound.
Mass of Compound Mass of Mn Mass of O
2.59 g 1.63 g 2.59 g - 1.63 g 0.96 g
1.63 g Mn ? 100 ___________________
___________________ 2.59 g compound
Mn
62.9 Mn
0.96 g O ? 100 _________________
_____________________ 2.59 g compound
O
37 O
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EMPIRICAL FORMULA CALCULATIONS
EMPIRICAL FORMULA The simplest whole-number
ratio of the atoms of different elements in a
compound
C6H6 C1H1
C6H12O6 C1H2O1
H2O H2O
Molecular Formula Empirical Formula
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Find the empirical formula of a compound that is
75.0 carbon and 25.0 hydrogen by mass.
1) Assume you have 100 g of the compound
? 75.0 g C and 25.0 g H
2) Calculate the moles of atoms of each element
x 1 mol C _____________________ 12.01 g C
6.24 mol C
75.0 g C
x 1 mol H ____________________ 1.01 g H
24.8 mol H
25.0 g H
3) Divide each number of moles by the smallest
number of moles
6.24 mol C _______________________ 6.24
24.8 mol H _______________________ 6.24
1.00 mol C
3.97 mol H
4) The integer mole ratio must be the atom ratio

CH4
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Find the empirical formula of a compound that is
30.4 nitrogen and 69.6 oxygen by mass.
4B-22
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Find the empirical formula of a compound that is
81.7 carbon and 18.3 hydrogen by mass.
x 1 mol C _____________________ 12.01 g C
6.80 mol C
81.7 g C
x 1 mol H ___________________ 1.01 g H
18.1 mol H
18.3 g H
6.80 mol C _______________________ 6.80
18.1 mol H _______________________ 6.80
1.00 mol C
2.66 mol H
? 2
? 2
2.00 mol C
5.32 mol H
If the moles of all elements are not within 0.1
moles of an integer, they must all be multiplied
by a constant number until they are integers
4B-23
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Find the empirical formula of a compound that is
81.7 carbon and 18.3 hydrogen by mass.
x 1 mol C _____________________ 12.01 g C
6.80 mol C
81.7 g C
x 1 mol H ___________________ 1.01 g H
18.1 mol H
18.3 g H
6.80 mol C _______________________ 6.80
18.1 mol H _______________________ 6.80
1.00 mol C
2.66 mol H
? 3
? 3
3.00 mol C
7.98 mol H
Empirical formula C3H8
If the moles of all elements are not within 0.1
moles of an integer, they must all be multiplied
by a constant number until they are integers
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MOLECULAR FORMULA CALCULATIONS
MOLECULAR FORMULA The actual number of the
atoms of different elements in a molecule
C1H1
Empirical Formula Molecular Formula
or C2H2
C1H1
or C3H3
or C4H4
etc.
4B-25
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Find the molecular formula of a compound that is
5.9 hydrogen and 94.1 oxygen by mass, and has a
molar mass of 34.0 g/mol.
x 1 mol H _____________________ 1.01 g H
5.8 mol H
5.9 g H
x 1 mol O _____________________ 16.00 g O
5.88 mol O
94.1 g O
5.8 mol H _______________________ 5.8
5.88 mol O _______________________ 5.8
1.0 mol H
1.0 mol O
Empirical formula HO
Find the molar mass of the empirical formula
4B-26
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HO 1 mol H (1.01 g/mol) 1.01 g 1 mol O
(16.00 g/mol) 16.00 g 17.01 g
Divide the compounds actual molar mass by the
empirical formulas molar mass it should be
very close to an integer
34.0 g/mol _________________________ 17.01 g/mol
2
? The molecular formula is 2 times the empirical
formula
Molecular formula H2O2
4B-27
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Find the molecular formula of a compound that is
26.7 P, 12.1 N, and 62.1 Cl by mass, and has a
molar mass of about 700 g/mol.
4B-28
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4B-29
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REVIEW FOR TEST
Polyatomic Ions Names and Formulas of Ionic
Compounds, Covalent Compounds, and Acids Atomic
Mass Mole, Avogadros Number Molar Mass of
Elements Conversions from Mass to Moles to
Atoms Molar Mass of Compounds Conversions from
Mass to Moles to Molecules Percentage Composition
by Mass Empirical Formula Calculations Molecular
Formula Calculations
4B-30