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
4B-1 (of 30)
20Ne
21Ne
22Ne
4B-2
19.99 u
20.99 u
21.99 u
90.92 %
0.26 %
8.82 %
Any sample of neon contains the same percentage of each isotope
ELEMENT’S ATOMIC MASS - Found on the Periodic Table, the average
atomic mass of all of the element’s naturally occurring isotopes
The atomic mass of neon is 20.18 u
The atomic mass of calcium is 40.08 u
4B-3
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
4B-4
1 Ne atom
20.18 u
20.18 g Ne
1 Ca atom
40.08 amu
40.08 g Ca
These two samples contain the same number of atoms
4B-5
The number of atoms contained in 20.18 g neon or 40.08 g calcium
(numerically the element’s 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
4B-6
Experimentally it has been shown that a mole is 6.022  1023 atoms
AVOGADRO’S NUMBER – 6.022  1023
For shoes:
1 pair
= 2 shoes
For eggs:
1 dozen = 12 eggs
For atoms:
1 mole
= 6.022  1023 atoms
= The number of atoms in 20.18 g Ne
= The number of atoms in 40.08 g Ca
MOLE – The number of atoms contained in 12 g of carbon-12
4B-7
The Element’s Atomic Mass
The Element’s Molar Mass
The average atomic mass of the
element’s isotopes, measured in
amu’s
The mass necessary to have 1 mole
of atoms of the element, measured in
grams
1 average O atom = 15.9994 u
1 mole O atoms = 15.9994 g
A Molar mass may be rounded to the hundredths place as long as it
doesn’t limit the number of significant figures in a calculation
4B-8
Calculate the mass of a sample containing 2.50 moles of phosphorus atoms
For phosphorus: 30.97 g P = 1 mol P
2.50 mol P x 30.97 g P
_____________
= 77.4 g P
1 mol P
Calculate the moles of atoms contained in a 10.0 g sample of potassium
4B-9
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
_____________________________
= 2.71 x 1024 atoms Na
1 mol Na
Calculate the moles of atoms in a sample of 1.50  1023 tungsten atoms
4B-10
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
1 mol Fe
_______________
55.85 g Fe
4B-11
x 6.022 x 1023 atoms Fe
_____________________________
1 mol Fe
= 1.08  1022 atoms Fe
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 Cl1 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
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)
22 mol H (1.01 g/mol)
= 144.12 g
= 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)
4B-13
Calcium hydroxide, Ca(OH)2
1 mol Ca (40.08 g/mol)
2 mol O (16.00 g/mol)
2 mol H (1.01 g/mol)
=
=
=
40.08 g
32.00 g
2.02 g
74.10 g
4B-14
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
3.50 mol MgF2 x 62.31 g MgF2
__________________
1 mol MgF2
4B-15

62.31 g MgF2 = 1 mol MgF2
= 218 g MgF2
Calculate the number of moles of zinc bromide present in a 10.0 g
sample of the compound.
4B-16
PERCENT COMPOSITION OF COMPOUNDS BY MASS
Calculate the percent composition by mass of iron (III) oxide.
Fe2O3
2 mol Fe (55.85 g/mol)
3 mol O (16.00 g/mol)
= 111.70 g
= 48.00 g
159.70 g
% Fe =
111.70 g Fe
________________________________
 100
=
69.944 % Fe
 100
=
30.06 % O
159.70 g Fe2O3
%O
=
48.00 g O
________________________________
159.70 g Fe2O3
4B-17
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
4B-18
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
2.59 g
Mass of Mn
1.63 g
Mass of O
% Mn =
2.59 g - 1.63 g = 0.96 g
1.63 g Mn
______________________________________
 100
=
62.9 % Mn
 100
=
37 % O
2.59 g compound
%O
=
0.96 g O
______________________________________
2.59 g compound
4B-19
EMPIRICAL FORMULA CALCULATIONS
EMPIRICAL FORMULA – The simplest whole-number ratio of the atoms of
different elements in a compound
4B-20
Molecular Formula:
C6H6
C6H12O6
H2O
Empirical Formula:
C1H1
C1H2O1
H2O
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
75.0 g C
x
1 mol C
_____________________
= 6.24 mol C
12.01 g C
25.0 g H
x 1 mol H
____________________
= 24.8 mol H
1.01 g H
3)
Divide each number of moles by the smallest number of moles
6.24 mol C = 1.00 mol C
_______________________
6.24
4)
24.8 mol H = 3.97 mol H
_______________________
6.24
The integer mole ratio must be the atom ratio:
4B-21
CH4
Find the empirical formula of a compound that is 30.4% nitrogen and
69.6% oxygen by mass.
4B-22
Find the empirical formula of a compound that is 81.7% carbon and
18.3% hydrogen by mass.
81.7 g C
x
1 mol C
_____________________
= 6.80 mol C
12.01 g C
18.3 g H
x 1 mol H
___________________
= 18.1 mol H
1.01 g H
6.80 mol C = 1.00 mol C  2
_______________________
6.80
18.1 mol H = 2.66 mol H  2
_______________________
6.80
= 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
Find the empirical formula of a compound that is 81.7% carbon and
18.3% hydrogen by mass.
81.7 g C
x
1 mol C
_____________________
= 6.80 mol C
12.01 g C
18.3 g H
x 1 mol H
___________________
= 18.1 mol H
1.01 g H
6.80 mol C = 1.00 mol C  3
_______________________
6.80
18.1 mol H = 2.66 mol H  3
_______________________
6.80
= 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
4B-24
MOLECULAR FORMULA CALCULATIONS
MOLECULAR FORMULA – The actual number of the atoms of different
elements in a molecule
Empirical Formula:
C1H1
Molecular Formula:
C1H1 or C2H2 or C3H3 or C4H4 etc.
4B-25
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.
5.9 g H
x
1 mol H
_____________________
= 5.8 mol H
1.01 g H
94.1 g O
x
1 mol O
_____________________
= 5.88 mol O
16.00 g O
5.8 mol H
_______________________
= 1.0 mol H
5.8
5.88 mol O = 1.0 mol O
_______________________
5.8
Empirical formula: HO
Find the molar mass of the empirical formula
4B-26
HO
1 mol H (1.01 g/mol)
1 mol O (16.00 g/mol)
=
=
1.01 g
16.00 g
17.01 g
Divide the compound’s actual molar mass by the empirical formula’s
molar mass – it should be very close to an integer
34.0 g/mol
_________________________
≈ 2
17.01 g/mol
 The molecular formula is 2 times the empirical formula
Molecular formula: H2O2
4B-27
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
4B-29
REVIEW FOR TEST
Polyatomic Ions
Names and Formulas of Ionic Compounds, Covalent Compounds, and Acids
Atomic Mass
Mole, Avogadro’s 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