Module 4.1:
The Mole Concept and
Molar Mass
Objectives
• to be able to define atomic mass unit
• to calculate the average atomic mass of elements
• to calculate the mass of a given number of moles of
an element or compound, or vice versa
• to calculate the mass of a given number of particles
of an element or compound, or vice versa
Atomic Mass
It is measured in atomic mass unit (amu), a
relative unit based on the value of exactly 12
for the C-12 isotope.
1 amu = 1/12 the mass of C-12 atom
Atomic Mass
The atomic mass of Cu-63 is 62.93 amu. This
means that relative to C-12, one atom of Cu63 is
62.93/12 or 5.2 times the
mass of C-12 atom
Exercises
One atom of Se-77 is 6.410 times as
heavy as an atom of C-12. What is
the atomic mass of Se-77?
Answers
One atom of Se-77 is 6.410 times as
heavy as an atom of C-12. What is
the atomic mass of Se-77?
6.410 x 12 amu = 77 amu
Average Atomic Mass
The average atomic mass takes into account
the different isotopes of an element and their
relative abundances. It is not a simple average
but a weighted average.
Average Atomic Mass
Isotopes of elements occur in different
abundances. Some are more abundant than
others.
Carbon has two isotopes. The natural
abundance of C-12 is 98.90% while that
of C-13 is 1.10%. The atomic mass of C13 is 13.00335 amu, while C-12 is 12
Average Atomic Mass
Average atomic mass of Carbon
= (atomic mass C-12)(% abundance C-12) +
(atomic mass C-13)(% abundance C-13)
= (12.00 amu)(0.9890) + (13.00335 amu)(0.0110)
= 12.01 amu
Exercises
Copper has two stable isotopes with the
following masses and abundances:
Cu-63 (62.93 amu, 69.09% abundance)
and Cu-65 (64.9278 amu, 30.91%
abundance).
Calculate the average atomic mass of
copper.
Answers
Copper has two stable isotopes with the
following masses and abundances:
Cu-63 (62.93 amu, 69.09% abundance)
and Cu-65 (64.9278 amu, 30.91%
abundance).
Calculate the average atomic mass of
copper.
Ans: 63.55 amu
Exercises
An element consists of an isotope with
mass of 10.0129 amu and 19.91%
abundance, and another isotope with
mass of 11.0093 amu and 80.09%
abundance. Calculate the average atomic
mass of the element and identify what
element is stated.
Answers
An element consists of an isotope with
mass of 10.0129 amu and 19.91%
abundance, and another isotope with
mass of 11.0093 amu and 80.09%
abundance. Calculate the average atomic
mass of the element and identify what
element is stated. Ans: 10.81 amu, Boron
Average Molecular Mass
The molecular mass is the sum of the
average atomic masses of the atoms in the
molecule. It is expressed in atomic mass unit,
and is numerically equal to the molar mass
(grams per mole) of the molecule.
Average Formula Mass
The formula mass is the sum of the atomic
masses of the atoms in the ionic compound.
It is also expressed in atomic mass unit, and
is also numerically equal to the molar mass
(grams per mole) of the substance.
Avogadro's Number
The mole (mol) is defined as
the amount of substance
containing the same number
of particles as there are
atoms in exactly 12g of C-12
isotope.
Avogadro's Number
One mole of substance is
equivalent to the Avogadro's
number of particles:
23
10
6.022 x
particles/mole
Molar Mass
The molar mass of a compound (molecular
or ionic) is the mass in grams of one mole of
a substance. It is numerically equal to the
molecular mass of formula mass.
Exercises
One mole of C-12 has a mass of exactly
12 grams and one mole of C-12 has
Avogadro's number of atoms. Calculate
the mass of one atom of C-12 in grams,
then calculate the mass in grams of 1
amu.
Answers
One mole of C-12 has a mass of exactly
12 grams and one mole of C-12 has
Avogadro's number of atoms. Calculate
the mass of one atom of C-12 in grams,
then calculate the mass in grams of 1
amu.
-23
-24
Ans: 1.993 x 10 g; 1.661 x 10 g
Conversions
÷
Particles
6.022 x 1023
×
÷
Moles
Molar Mass
×
Mass
Short Assessment
1. How much heavier is an atom of Br
relative to an atom of C?
2. Which element has an average atomic
mass that is about ten times that of
fluorine?
Short Assessment
Element A consists of isotope A-6 with natural
abundance of 7.5% and a mass of 6.0151 amu, and
isotope A-7 with natural abundance 92.5 and mass
of 7.0160 amu.
3. Calculate the average atomic mass of
Element A.
4. Identify Element A.
Short Assessment
Naphthalene as a molecular formula of
C8H10.
5. What is the molecular mass of
naphthalene?
6. What is
naphthalene?
the
molar
mass
of
Short Assessment
7. Which will have a higher molar mass:
0.500 mol zinc, or 0.250 mol lead?
8. What is the molar mass of lithium
carbonate?
Short Assessment
9. A bottle of Calcium supplements in
tablet form contains 268 g Ca. How many
atoms are present in 268 g Calcium?
10. How many moles of Copper are there
in 875 g Cu?
Short Assessment
You have 10
minutes to
answer.
Answers
1. 6.653 times
2. Osmium, Os
3. 6.94 amu
4. Lithium, Li
5.106.16 amu
6. 106.16 g/mol
7. 0.250 mol Pb
8. 73.89
9. 4.03 x 1024 atoms
10. 13.8 mol