INTRODUCTION TO
THE MOLE
Mrs. Pesko
In the Hot Seat
• What words can you think of that stand for a set
number of things?
In the Hot Seat
• Eggs come in packages of 12, which is given a
unit called a dozen. Paper is not packaged by
the dozen. Paper is packaged by a ream. A
ream of paper has 500 sheets. Why is it useful
to use units like a dozen or a ream?
In the Hot Seat
• Suppose you work in a candy store that sells
gourmet jelly beans. People come in and ask
for 50 beans, 100 beans, 1000 beans, and so
on. As you can imagine, this process can be
tedious and time consuming. What is a better
system that you can implement that would save
time in completing a customer’s order?
In the Hot Seat
• Atoms and molecules are extremely
small. How do you think chemists’ “count”
them?
Counting Units of Measurement
• Many items require exact counting units
• One dozen = 12 items
• One baker’s dozen = 13 items
• A pair = 2 items
• One gross = 12 dozen = 144 items
• One ream = 500 items
• Chemists also have a unit that is used to count atoms or
molecules!
The MOLE!
Introducing the Mole as a Unit of Measure
• Remember, a mole is
the SI unit that describes
the amount of a
substance
• One mole = 6.022 x 1023
objects
• In chemistry, the objects
are atoms, molecules,
ions, and formula units
• 1 mole = 6.022 x 1023
chemical particles
• This number is called
Avogadro’s Number, NA
How Big is Mole?
How Big is a Mole?
The Mole as a Unit of Measure
• Why would we ever use such a large unit as the mole?
• Because chemical particles ARE
tiny
• Now, why is the mole THAT big number of 6.022 x 1023?
• Scientists chose a standard to compare ALL atoms to
• That standard is carbon!
• Specifically, a mole was defined as the number of atoms
contained in exactly 12 grams of pure carbon-12
• The number of atoms contained in exactly 12 grams of carbon-
12 was determined to be 6.02214 x 1023 atoms
• Name given to this HUGE number is a “mole” (stems from
the word “molecule”)
More on the Mole
• So, this means that…
• 1 mole of 12C atoms = 6.022 x 1023 atoms
• 1 mole of 16O atoms = 6.022 x 1023 atoms
• 1 mole of 35Cl atoms = 6.022 x 1023 atoms
• 1 mole of anything = 6.022 x 1023 anything
• Remember, a mole is NOT a weight (mass)!
• It is a COUNT
SO, HOW BIG IS THE
MOLE?
Let‘s Do a Little Activity to Help Us Understand!
Turn to Page
But Wait…Atoms are Too Small to Be
Counted!
• This is a very true statement
• So, how do we determine the number of items when we
can’t directly count them?
• We weigh them in a bigger sample!
• Therefore, you must be able to convert a count (how
many) to a mass (how much)
• A Gumball Example:
• A candy shop keeper knows that 10 gumballs have a mass of 21.4
g. What is a possible method to measure out 200 gumballs without
actually counting the individual gumballs?
Counting by Weighing – A Chemical
Example
• Recall that 12 grams of 12C = 6.022 x 1023 atoms (exactly)
• Therefore, one could calculate the mass of a single 12C atom:
12 g 12C
−23 g
=
1.992647
×
10
atom 12C
6.022 × 1023 atoms
1 atom of 12C = 1.992647 × 10−23 g
• We just converted from a count (how many) to a mass (how
much)
• Did you see that the mass is a really tiny number?
• To avoid working with such impossibly small numbers, a new unit was
devised called the atomic mass unit (amu)
What is an Atomic Mass Unit?
• Now, remember the mass of an atom is largely derived
from the masses of the protons and the neutrons in the
nucleus
• In carbon-12, there are 6 protons and 6 neutrons
•
Proton and neutron mass are essentially equal
• Therefore, 1 amu is essentially the mass of a single proton or
neutron
• 1 amu is defined as 1 /12 the mass of a single carbon-12
atom
• So, the atomic mass of carbon-12 is 12 amu
1 amu = 1
12th
−23 g
1.992647
×
10
the mass of a 12C atom =
= 1.661 × 10−24 g
12
More on Atomic Mass
• Look on your periodic table at the mass of carbon
• It says 12.01 NOT 12!
• Why doesn’t the atomic mass given on the periodic table
match the atomic mass we just calculated??
• Are all carbon atoms exactly the same?
• No, there are isotopes!
• Isotopes have different masses because of different number of neutrons
in the nucleus!
Average Atomic Mass
• So, the element carbon on the periodic table is actually a
mixture of the isotopes carbon-12, carbon-13, and
carbon-14
• Therefore, the atomic mass on the periodic table is a weighted
average of the atomic mass of all isotopes of that particular
element
• Come to find out, objects do not need to have identical
masses to be counted by weighing
• Simply need to know the average mass of the objects
• For purposes of counting, the objects behave as though they were
all identical
COUNTING BY WEIGHING
PRACTICE!
Work on “A Mole of Pennies” WS
How Do You Calculate Average Atomic
Mass?
• One can calculate the average atomic mass of an
element if the abundance of each isotope for that element
is known
Average Atomic Mass
% natural abundance
=
∙ atomic mass1
100
1
% natural abundance
+
∙ atomic mass2 …
100
2
Practice!
• Silicon exists as a mixture of three isotopes. Determine
its average atomic mass based on the following data:
Isotope
28Si
29Si
30Si
Mass (amu)
Abundance
27.9769265
28.9764947
29.9737702
92.23 %
4.67 %
3.10 %