The Mole
Counting by Weighing
What are some thing that are
sold by weight?
Why are some things sold by
weight?
Because it is more
practical to weigh
out a bunch of
small pieces than it
is to count them
individually.
Do you really think someone counts out
every individual tootsie roll?
Back to Chemistry…
• Atoms are so small that we work with
billions of them at a time in the laboratory.
• We can’t count them out individually, but
we can count them by weighing.
Mole
Mole
Naked Mole Rat
What is a mole?
It's not a spy, a machine for digging tunnels, a
burrowing animal, or a spot of skin pigmentation
it's
The Chemical Mole
a unit of measurement containing
23
6.02 x 10
1 pair of atoms is
2 atoms!
1 dozen atoms is
12 atoms!
1mole of atoms is
602,000,000,000,000,000,000,000
atoms!
Avogadro Number
6.022 x
23
10
How BIG is 6.022 x
23
10 ?
Could Bill Gates ever become a
“moleonaire”?
Bill Gates only currently has
$ 70 billion ($70 x 109)
To become a moleonaire Bill
Gates would have to earn
$ 33,000 trillion a day for the
next 50 years!
Not likely!
Too bad Bill! Soo sad ):
How much water is there in the
Pacific Ocean?
• Approximately 6 x 1020 L
• which is equivalent to 6 x 1023 mL !
• The Pacific Ocean contains a mole of mL of
water
A Galactic Analogy
• A mole of periods (.) lined up side by side
would equal the radius of our galaxy
(30,000 light years!)
The “chemist’s dozen”
• Think of moles as a "chemist's dozen". Just as
12 eggs is a dozen eggs, 6.02 x1023 eggs is a
mole of eggs. 6.02 x 1023 molecules of oxygen
is a mole of oxygen.
• The number of grams in a mole is different
from substance to substance.
• Picture it this way: a dozen elephants have a
different weight than a dozen rabbits- but in
each case, you have a dozen animals.
Similarly, a mole of oxygen gas has a different
weight than a mole of water- but in each case,
you have 6.02x1023 molecules.
Why use Moles?
• Often want to know how many molecules you
have in a sample of a substance.
• Counting the molecules individually would be
completely impractical.
• Even if you had a way to see the individual
molecules, there are just too many, even in a
tiny sample.
• Moles were defined to solve the problem of
counting large numbers of molecules.
• With moles, you count the number of
molecules in the sample by finding the mass.
What can we count with dozen?
• What the items are doesn't matter.
• You can have a dozen bagels, a dozen
donuts, a dozen molecules, a dozen
ions, or a dozen of stars.
• The number of items in a dozen will
always be 12.
What can we count with moles?
• Again, what the items are doesn't matter.
• You can have a mole of molecules, a mole
of ions, or a mole of stars.
• The number of items in a mole will always
be 6.02 x 1023 .
• This number is known as Avogadro's
number.
Why such a large number for the mole?
• Well, why do they sell eggs in a dozen?
– Maybe because no one wants to buy just one egg, and
if you buy fifty, some will go bad before you eat them.
• The reason we need so many items in a mole may
be because we need to group molecules in very
large groups in order to be able to get a
measurable reading on our balances.
– We can't find the mass of one atom, or even one gross
of atoms, on our laboratory balances, the instruments
are not sensitive enough. We can, however, find the
mass of one mole of atoms on our balance.
Moles of atoms
•
•
•
•
1 mole C = 12.0 g C
1 mole B = 10.8 g B
1 mole Cu = 63.5 g Cu
1 mole element = (atomic mass) g element
Molar Mass
Molar mass is the
mass of one mole of a
substance.
The units for molar
mass are grams per
mole (g/mol)
Determining Molar Mass
• Determine the total number of each type
of atom present in the compound.
• Find the molar mass of each type of atom
from the periodic table.
• Multiply the molar mass of each specific
type of atom by the number of that type of
atom.
• Add the total molar mass for each type of
atom.
Sample 1
Find the Molar Mass of Water.
Water = H2O
H = 2 moles H x 1.00 g/mol H
O = 1 mole O x 16.0 g/mol O
= 18.0 g/mol H2O
Sample 2
Find the Molar Mass of glucose.
glucose = C6H12O6
C =
H =
O =
6 moles C x 12.0 g/mol C
12 mole H x 1.00 g/mol H
6 mole O x 16.0 g/mol O
= 180 g/mol C6H12O6
Sample 3
Find the Molar Mass of
Aluminum Hydroxide,
Al(OH)3.
Al = 1 moles Al x 27.0 g/mol Al
O = 3 mole O x 16.00 g/mol O
H = 3 mole H x 1.0 g/mol H
= 78 g/mol Al(OH)3
Sample 4
Find the Molar Mass of
Magnesium Acetate
Mg(C2H3O2)2.
Mg = 1 moles Mg x 24.3 g/mol Mg
C = 4 mole C x 12.00 g/mol C
H = 6 mole H x 1.0 g/mol H
O = 4 mole O x 16.0 g/mol O
= 142.3 g/mol Mg(C2H3O2)2
Sample 5
Find the Molar Mass of
Ammonium Sulfate,
(NH4)2SO4.
N=
H =
S =
O =
2 moles N x 14.0 g/mol N
8 mole H x 1.00 g/mol H
1 mole S x 32.0 g/mol S
4 mole O x 16.0 g/mol O
= 132 g/mol (NH4)2SO4
Molar Conversions
Use Dimensional
Analysis!
Sample 1
How many moles are there in 0.028 g of
iron?
0.028g
x 1 mole Fe
56.0 g Fe
= .0005 mole Fe
5.00 x 10-4
mole Fe
Sample 2
How many atoms are there in 0.028 g of iron?
.028g
x 1 mole Fe
56.0 g Fe
x 6.02 x 1023 atoms
1 mole
3.02 x 1020
Fe atoms
Sample 3
How many grams are represented by
0.500 mole of AgCl
0.500 mole
x 143 g AgCl
1 mole
= 71.45 g
71.5 g AgCl
Sample 4
How many atoms are in 5.03g of Mercury?
5.03 g Hg
x 1 mole
200.0 g
x 6.02 x 1023 atoms
1 mole
= 1.51 x 1022
1.51 x 1022
Mercury
atoms