INTRODUCTION TO
THE MOLE
Ms. Grobsky
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!
Introducing the Mole as a Unit of Measure
• 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 and
molecules
• 1 mole = 6.022 x 1023
atoms (molecules)
• Avogadro’s Number, NA
The Mole is a Unit!
A Mole is a Unit Music Video
Exactly How Big is a Mole?
• A mole of marshmallows would cover the planet Earth 12 miles
high
• A mole of marbles would fill the entire Grand Canyon
• There would still be enough left over to displace all the water in Lake
Michigan and a few other lakes!
• Computers can count at the rate of over 800 million counts per
second
• At this rate it would take a computer over 25 million years to count to 6.02
x 1023
• A mole of hockey pucks would be equal to the mass of the Moon.
• Assume that each human being has 60 trillion body cells (6.0 x
1013) and the Earth's population is 7 billion (7 x 109)
• The total number of living human body cells on the Earth at the present
time is 3.6 x 1023 or a little over half of a mole
How Big is Mole?
How Big is a Mole?
WORK ON HOW BIG IS A
MOLE WORKSHEET!
The Mole as a Unit of Measure
• But why would we ever use such a large unit as the mole?
• Because ATOMS ARE
tiny
• Atoms are too small to count individually
• So, how do we determine the number of items when we can’t
directly count them?
• We weigh them in a bigger sample!
• 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 – 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?
RELATING MASS OF
ATOMS TO THE MOLE
Mind Catalyst Wrap-Up
• With the Mind Catalyst, we observed that one mole of
different substances look different
• Now we will begin to explore the unique mass of
substances and compare the mass of one mole of
different substances
WHY is the Mole THAT Big Number?
• 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,
• 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
But Wait…I Thought Atoms are Too Small
to Be Counted??
• This is a very true statement
• Therefore, you must be able to convert a count (how
many) to a mass (how much)
• 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
Wow! That’s 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?
• Defined as one 12th the mass of a single carbon-12 atom
−23 𝑔
1.992647
×
10
1 amu = 1
the mass of 𝑎 12𝐶 𝑎𝑡𝑜𝑚 =
th
12
12
−24
= 1.661 × 10 𝑔
• Now, remember the mass of an atom is largely derived
from the masses of the protons and the neutrons in the
nucleus
• There are 6 protons and 6 neutrons in C-12
•
Proton and neutron mass are essentially equal
•
•
Therefore, 1 amu is essentially the mass of a single proton or neutron
As a result, the atomic mass of carbon-12 is 12 amu
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??
• Because the element in the periodic table is 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
Atomic Mass vs. Molar Mass
• More useful to associate atomic mass in
amu with a mass in grams
• We know that by definition a mole of
carbon-12 atoms weighs exactly 12 grams
• Therefore, to measure out a mole’s worth of
carbon-12 atoms, you would weigh out exactly
12.00 g of carbon-12
• This number is called the molar mass
• The mass of a mole of atoms in grams
• Units are grams/mole (g/mol)
For any element, atomic mass (in amu) =
molar mass (in grams)
In Summary…
1 atom of 12C = 12 amu
1 mole of 12C atoms = 12 g
12 g of 12C = 6.022 x 1023 atoms
Molar Mass from the Periodic
Table
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
Practice – Determining the Molar Mass of
an Element
• What is the molar mass of the following elements?
• Strontium
• Silver
• Tellurium
• Lithium
• Chlorine
Determining the Molar Mass of a
Compound
• Scientists also use the Periodic Table to determine the
molar mass (also known as the formula weight) of
compounds
• The molar mass of the compound is the sum of the
molar masses of each element times the number of
atoms of that element in the compound
22
Guide to Calculating Molar mass
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
Practice!
• Calculate the molar mass of CaCl2.
Element
Number
of Moles
Molar Mass
Total Mass
Ca
1
40.08 g/mol
40.08 g
Cl
2
35.45 g/mol
70.90 g
CaCl2
110.98 g
Practice!
Calculate the molar mass for Al(OH)3 with four
significant figures.
1) 44.00 g/mol
2) 75.00 g/mol
3) 78.00 g/mol
Solution
STEP 1 Obtain the molar mass of each element.
Al 26.98 g/mol
O 16.00 g/mol
H 1.008 g/mol
STEP 2 Multiply each by the subscript.
1 mol Al x 26.98 g Al = 26.98 g
1 mol Al
3 mol O x 16.00 g O
= 48.00 g
1 mol O
3 mol H x 1.008 g H
= 3.024 g
1 mol H
STEP 3 Calculate the molar mass by adding the
masses of the elements.
1 mol of Al(OH)3 = 78.00 g
26
Some One-Mol Quantities
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
USING DIMENSIONAL
ANALYSIS TO CALCULATE
MASS, MOLES, AND # OF
PARTICLES
Calculating with Moles
• As we have seen so far, the amount of a substance and
its mass is related
• Therefore, it is often useful to convert from grams to moles to
particles!
• You can use conversion factors to relate units (aka
dimensional analysis)!
• Depending on which conversion factor you ruse, you can solve for
either the amount of a substance (in particles or moles) or its mass!
General Format with Dimensional
Analysis
Unit to Keep
Unit to Keep
Given Unit ×
×
× …
Unit to Discard
Unit to Discard
= Wanted Unit
Molar Mass Conversion Factors
• Relate grams and moles of an element or compound
• Derived from EQUIVALENT STATEMENTS of molar mass
• Example:
1 mole CH4 = 16.04 g
• Each equivalent statement produces TWO conversion factors
• Example:
1 mol CH4
16.04 g
OR
16.04 g
1 mol CH4
• You now need to decide which conversion factor you should use in
your calculation
• Choose the one that will help you discard the unwanted unit!
Practice!
• Determine the mass in grams of 5.50 mol of iron
• Given:
• 5.50 mol Fe
• 1 mole Fe = 55.85 g
• This is your equivalent statement
•
1 mol Fe
55.85 g
OR
55.85 g
1 mol Fe
• Since I want to calculate GRAMS of iron, I need to use
the conversion factor that has grams in the numerator!
• Final calculation looks something like this:
55.85 g Fe
5.50 mol Fe ×
= 307 g Fe
1 mol Fe
Guide to Calculation Using Molar Mass
A Handy Guide for Converting Between
Mass and Moles
Moles are the
HEART of
chemistry!
Converting Between Mass, Moles, and
Particles
TIME FOR PRACTICE!