Chemistry Chapter 4 Student Notes
Lesson 2 Molar Mass (pg. 131 – 136)
A “MOLE” = 6.02 x 1023 of anything; atoms, candy bars, beans, etc.
ATOMIC MASS
Because of the very small “actual” mass of a single atom, the mass of elements is defined in the
following way:
Atomic Mass = Mass of 1 mole of atoms of that element
Therefore, when chemists speak of the “mass” of an element we are usually referring to the mass
of one mole (6.02 x 1023) of atoms. For Carbon the mass of one mole is 12 grams:
a.k.a.
Molar mass of Carbon
Carbon = 12.0 g/mole
(as shown on the periodic table)
Said another way, the gram atomic mass of carbon = 12 g/mole
IMPORTANT FACT
The MASS # on the PERIODIC TABLE is
the average Molar Mass for each individual element
MOLECULAR MASS
Because of the very small “actual” mass of a single molecule, the mass of a molecular compound
is defined in the following way:
Molecular Mass = Mass of 1 mole of the molecules
When chemists speak of the “mass” of a molecular compound we add the mass of a moles’ worth
(6.02 x 1023 atoms) of the total number of each atom in the molecule.
For sucrose (table sugar) we have the molecular formula C12H22O11 and the mass is 342 grams:
a.k.a.
Molar mass of sucrose
Carbon =
+ Hydrogen =
+ Oxygen =
=
12 atoms x 12.0 g/mole Carbon contributes 144g
22 atoms x 1.01 g/mole Hydrogen adds 22g
11 atoms x 16.0 g/mole Oxygen adds 176g
342 g/mole
Most accurately, we would say that the gram molecular mass of sucrose = 342 g/mol
(i.e., a mole of sucrose molecules has a mass of 342.3 g)
Chemistry Chapter 4 Student Notes – cont’d
FORMULA MASS
Because of the very small “actual” mass of a single formula unit of an ionic compound, the mass
of ionic compounds is defined in the following way:
Formula Mass = Mass of 1 mole of the formula unit
When chemists speak of the “mass” of an ionic compound we add the mass of a moles’ worth
(6.02 x 1023 atoms) of the total number of each atom in the formula unit of the compound.
For sodium chloride (table salt) we have the formula NaCl and the mass would be 58.5 grams:
a.k.a.
Molar mass of
sodium chloride
Sodium =
+ Chloride =
=
1 atom x 23.0 g/mole
1 atom x 35.5 g/mole
Sodium contributes 23g
Chloride adds 35.5g
58.5 g/mole
Most accurately, we would say that the gram formula mass of sodium chloride = 58.5 g/mol
(i.e., a mole of Sodium Chloride has a mass of 58.5 g)
Chemistry Chapter 4 Student Notes – cont’d
Problems: Gram Atomic Mass
What is the Mass (in grams)of:
Helium Atoms (He) (atomic mass of Helium = 4.00 g/mole)
1 mole:
2 moles:
8 moles:
Iron Atoms (Fe) (atomic mass of Iron = 55.8 g/mole)
1 mole:
2 moles:
8 moles:
Problems: Gram Molecular Mass
What is the Mass (in grams)of:
Hydrogen Molecules (H2)
note: the periodic table shows the atomic mass of a single Hydrogen atom = 1.01 g/mole
1 mole:
2 moles:
8 moles:
Oxygen Molecules (O2)
note: the periodic table shows the atomic mass of a single Oxygen atom = 16.0 g/mole
1 mole:
2 moles:
8 moles:
Sugar (sucrose) Molecules (C12H22O12)
(the atomic mass of H and O are listed above, Carbon = 12.0 g/mole)
1 mole:
2 moles:
8 moles:
Chemistry Chapter 4 Student Notes – cont’d
Problems: Gram Formula Mass
What is the Mass (in grams) of :
One mole of KMnO4
K = _______ g/mole
+ Mn = ________ g/mole
+ O = ________ g/mole x 4 Oxygens in KMnO4 = ________ g/mole
adding them all together = ________ g per formula unit
One mole of KCl
One mole of Na2SO4
Two moles of Sodium Chloride
Four moles of Iron II Oxide
Chemistry Chapter 4 Student Notes – cont’d
Problems: Gram Atomic Mass
Helium Atoms (He) (atomic mass of Helium = 4.00 g/mole)
1 mol
4g
=4g
1 mol
2 mol
4g
=8g
1 mol
8 mol
4g
= 32 g
1 mol
Iron Atoms (Fe) (atomic mass of Iron = 55.8 g/mole)
1 mol
55.8 g = 55.8 g
1 mol
2 mol
55.8 g = 112 g
1 mol
8 mol
55.8 g = 446 g
1 mol
Problem: Gram Molecular Mass
What is the Mass in grams of this many moles of…
Hydrogen Molecules (H2) (atomic mass of Hydrogen = 1.01 g/mole)
1 mole H2
2 mol H
1 mol H2
1.01 g = 2.02 g
mol H
2 mole H2
2 mol H
1 mol H2
1.01 g = 4.04 g
mol H
8 mole H2
2 mol H
1 mol H2
1.01 g = 16.2g
mol H
Chemistry Chapter 4 Student Notes – cont’d
Oxygen Molecules (O2) (atomic mass of Oxygen = 16.0 g/mole)
1 mole O2
2 mol O
1 mol O2
16.0 g = 32.0 g
mol O
2 mole O2
2 mol O
1 mol O2
16.0 g = 64.0 g
mol O
8 mole O2
2 mol O
1 mol O2
16.0 g = 256 g
mol O
Sugar (sucrose) Molecules (C12H22O12) (atomic mass of H and O listed above, Carbon = 12.0 amu)
12 C’s x 12 g/mol = 144 g/mol
22 H’s x 1.01 g/mol = 22 g/mol
12 O’s x 16.0 g/mol = 192 g/mol
= 358 g/mol of C12H22O12
1 mole: 358 g
2 moles: 716 g
8 moles: 2,860 g
Chemistry Chapter 4 Student Notes – cont’d
Problem: Gram Formula Mass
What is the Mass (in grams) of :
One mole of KMnO4
K=
+ Mn =
+ O = 16.0 g/mole x 4 Oxygen’s in KMnO4 =
adding them all together =
39.1
54.9
64.0
158.0
One mole of KCl
K = 39.1 g/mole
+ Cl = 35.5 g/mole
= 74.6 g
One mole of Na2SO4
Formula = Na2SO4
2 x 23.0
two Na’s
46
+ 32.1
one S
+ 4 x 16.0
four O’s
+ 32
+ 64
Two moles of Sodium Chloride
Formula = NaCl
23.0 + 35.5 = 58.5 g/mole NaCl
2 mol 58.5 g = 117 g NaCl
1 mol
Four moles of Iron II Oxide
Formula = FeO
55.8 + 16.0 = 71.8 g/mole FeO
4 mol 71.8 g = 287 g FeO
1 mol
= 142 g/mole Na2SO4
g/mole
g/mole
g/mole
g/mole