What is a Mole in
Chemistry? (Chapter 10)
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Today’s Learning
Objectives
1. Understand what a mole is in chemistry
2. Know the relationship of Avogadro’s
number to the mole
3. Be able to calculate molar mass
(molecular weight) of an element and a
compound
* An appreciation for how big Avogadro’s
number really is*
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What if you had …
2 pairs of shoes = 4 shoes
3 dozen eggs = 36 eggs
Do you know how many eggs are in a flat? In a crate?
mole
Do you know how many carbon atoms are in a _______?
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Avogadro’s Number
23
6.02 X 10
Do you know how many carbon atoms are in a mole?
Do you know how many protons are in a mole?
Do you know how molecules of H2O are in a mole?
Do you know how many footballs are in a mole?
It’s a big number!!!
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- It’s 602 billion trillion
602,000,000,000,000,000,000,000
- 6.02 X 1023 (in scientific notation)
Amedeo Avogadro
1776-1856
His most important contribution to chemistry is known as
Avogadro’s Law, which states that all gases that are at the
same volume, temperature, and pressure contain the same
number of molecules. This law allows chemists to calculate
the molecular weight of gases, and if the molecular formula
is known, the weight of each atom in the gas. Though it was
not well received at the time of publication, it was verified
by various other scientists after his death. Any chemist will
agree that Avogadro’s Law was a massive step forward in
the evolution of chemistry.
How was this number
discovered?
It had been estimated by several scientists over
the years with crude experiments, but in 1911
Robert Millikan did his famous oil drop
experiment which determined the charge on a
single electron. For years scientists had known
the charge on a mole of electrons, so a simple
calculation led to Avogadro’s number.
Scientific American article about discovery
of Avogadros number
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Just How Big is a Mole?
• Enough soft drink cans to cover the
surface of the earth to a depth of
over 200 miles.
• If you had Avogadro's number of
unpopped popcorn kernels, and
spread them across the United
States of America, the country would
be covered in popcorn to a depth of
over 9 miles.
• If we were able to count atoms at the
rate of 10 million per second, it
would take about 2 billion years to
count the atoms in one mole.
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The Mole
• 1 dozen cookies = 12 cookies
• 1 mole of cookies = 6.02 X 1023 cookies
• 1 dozen cars = 12 cars
• 1 mole of cars = 6.02 X 1023 cars
• 3 dozen Al atoms = 36 Al atoms
• 3 mole of Al atoms = 3 x 6.02 X 1023 atoms
Note that the NUMBER is always the same,
but the MASS is very different!
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Molar Mass (Molecular Weight)
• Defined as: mass of 1 mole of element or
molecule (in grams)
• Equal to the atomic mass from periodic table.
Easy!!!
1 mole of C atoms
=
12.0 g/mol
1 mole of Mg atoms
=
24.3 g/mol
1 mole of Cu atoms
=
63.5 g/mol
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Learning Check!
Find the molar mass
(usually we round to the tenths place)
A. 1 mole of Br atoms =
B. 1 mole of Sn atoms =
79.9 g/mole
118.7 g/mole
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Molar Mass of Compounds
Mass in grams of 1 mole of a compound equals the
sum of the molar mass of each element in that
compound
What is the molar mass of CaCl2? Or written in another
way …. How much does 1 mole of CaCl2 weigh?
CaCl2 = 1 Ca + 2 Cl
= 1 x 40.1 g/mol + 2 x 35.5 g/mol
= 40.1 g/mol + 71.0 g/mol
= 111.1 g/mol
Learning Check!
A. Molar Mass of K2O = ? Grams/mole
B. Molar Mass of antacid (Tums or
Rolaids) Al(OH)3 = ? Grams/mole
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Cool Down!!!!
Prozac (flouxetine),
C17H18F3NO, is a widely
used antidepressant that
inhibits the uptake of
serotonin by the brain.
Find its molar mass.