The Mole
Pay Attention this is really important!
What is “The Mole?”
It’s not this…
Or this…
And we’re not talking about food either…
We’re talking about this…
The Mole Defined
• The mole is the standard method in chemistry
for communicating how much of a substance
is present.
• Here is how the International Union of Pure
and Applied Chemistry (IUPAC) defines
"mole:"
• The mole is the amount of substance of a system
which contains as many elementary entities as
there are atoms in 0.012 kilogram of carbon-12.
When the mole is used, the elementary entities
must be specified and may be atoms, molecules,
ions, electrons, other particles, or specified
groups of such particles. This is the fundamental
definition of what one mole is. One mole contains
as many entities as there are in 12 grams of
carbon-12 (or 0.012 kilogram).
• In one mole, there are 6.022 x 1023 atoms.
Here's another way: there are 6.022 x
1023 atoms of carbon in 12 grams of carbon-12
Holy Smokes! That is a lot of zeroes…and that
number actually has a name.
Avogadro’s Number
• Never call it Avocado’s Number. Remember,
we’re not talking about guacaMOLE here, just
the mole.
• 6.022 x 1023 is such an important number that
it has its own name and symbol, N.
• Named Avogadro’s Number in honor of Italian
chemist who made critical contributions to
measuring atomic weights.
The Mole Applied
• In one mole of ANYTHING there are 6.022 x
1023 entities.
So…
One mole of donuts contains 6.022 x 1023
donuts
One mole of carbon atoms contains 6.022 x 1023
of carbon atoms
One mole of water molecules contains 6.022 x
1023 of H2O molecules.
Molecular Weight (Mass)
• Molecular Weight = weight in atomic mass
units of all the atoms in a given formula.
• Symbol = was and is most commonly amu but
not the most correct symbol is u.
• Why do we need to know molecular weight? It
is needed in order to know how many grams
are in one mole of a substance.
Four Steps to Calculating a substance’s
molecular weight:
• Determine how many atoms of each different
element are in the formula.
• Look up the atomic weight of each element in
a periodic table
• Multiply step one times step two for each
element
• Add the results of step three together and
round off as necessary.
Practice
• KCl
1. K = 1 Cl = 1
2. K = 39.10 Cl = 35.45
3. 1(39.10) + 1(35.45) =
4. 74.55 amu represents the molecular weight
of KCl
More Practice
•
•
•
•
•
H2O2
H=2 O=2
H = 1.00 O = 15.99
2(1) + 2(15.99) =
2+32 = 34 amu
Molar Mass
• Counting atoms or molecules is very difficult
since they are so small BUT you can “count”
atoms or molecules by weighing large
amounts of them on a balance.
• When we weigh one mole of a substance on a
balance, this is called “molar mass” and has
the units g/mol (grams per mole).
Molar Mass
• A molar mass is the weight in grams of one
mole
• One mole contains 6.022 X 1023 entities.
Therefore, a molar mass is the mass in grams
of 6.022 X 1023 entities
How to Calculate Molar Mass
• How do YOU calculate molar mass? The molar
mass IS the molecular weight (!) in grams (not
amu’s)
• So You calculate the molecular weight and
stick the unit “g/mol” after the number and
that is the molar mass for the substance in
question.
Example of Molar Mass
• Al(NO3)3
• (1xAl)+(3xN)+(9xO)  (1x 26.98)+(3x14.007) +
(9x 16.00) = 213.00 g/mol
• 213 grams is the mass of one mole of
aluminum nitrate
• 213 trams of aluminum nitrate contains 6.022
x 1023 entities of Al(NO3)3
Practice
AlCl3
Converting Moles to Grams
• The mole is the standard measurement
amount in chemistry and when substances
react with one another they do so in ratios of
moles. Problem is…balances give readings in
grams, not moles!
• So, when comparing one substance to another
using moles….we have to convert moles to
grams since this is the unit we measure from
balances.
The Steps
• 3 steps to converting moles of a substance to
grams:
• Determine how many moles are given in the
problem
• Calculate the molar mass of the substance
• Multiply step one by step two.
The Equation
• The equation looks like this:
• Grams of the substance = molar mass of the substance in grams
moles of the substance
one mole
• You replace the “grams of the substance” with
X because this is what you are solving for.
Example Problem
Problem: Calculate how many grams are in
0.700 moles of H2O2
• Determine how many moles you have ---0.700 moles
• Determine the molar mass of the substance --- H2O2 has a molar mass of 34.0146 g/mol
• Multiply the moles given by the substance’s
molar mass:
0.700 mole x 34.0146 g/mol = 23.8 grams
Significant Figures
• The answer of 23.8 trams has been rounded
to three significant figures because the 0.700
value had the least number of significant
figures in the problem.
• Using the equation – this problem would look
like:
x
=
34.0146 g
0.700 mol
1.00 mol
(then cross multiply and divide to solve for x)
More practice
Problem #2: Convert 2.50 moles of KClO3 to grams.
Converting Grams to Moles
The mole is the standard measurement of amount.
However, remember that balances do NOT give readings in
moles, they give readings in grams. Oh NO!!
Steps to convert grams of a substance to moles
• Determine how many grams are given in the
problem
• Calculate the molar mass of the substance.
• Divide step one by step two.
This might look familiar…
• Here is the equation:
• Grams of the substance = molar mass of the
substance in grams
moles of the substance one mole
• Where the “moles of the substance” will be
the unknown (x) and what you solve for.
Let’s Practice
• Convert 25.0 grams of KMnO4 to moles
• Determine how many grams are present --25.0 grams
• Determine the molar mass of the substance --KMnO4 is 158.034 g/mol
• Divide the grams given by the substance’s
molar mass
25.0 g
= 0.158 mol
158.034 g/mol
• If this problem were set up like the
proportion, it would look like this:
• 25.0 g =
158.034 g
x
1.00 mol
•
• Then, you cross-multiply and divide to solve
for X.
And one more practice problem…
Calculate how many moles are in 17.0 grams of
H2O2
Calculating The Mass of One
Molecule
• When calculating the mass of one molecule, do the
following:
• Calculate the molar mass of the substance
• Divide it by Avogadro's Number
• Make sure you have a periodic table and a calculator
handy.
• the technique to calculate the mass of one atom of
an element is exactly the same as for calculating the
mass of one molecule of a compound.
Practice problem #1
• What is the mass of one molecule of H2O?
• Step #1 - the molar mass of water is 18.015
g/mol. This was calculated by multiplying the
atomic weight of hydrogen (1.008) by two and
adding the result to the weight for one oxygen
(15.999).
• Please remember that you need the molar
mass first when trying to find the mass of one
molecule.
• Step #2 - divide the substance's molar mass by
Avogadro's Number.
• 18.015 grams/mol ÷ 6.022 x 1023 mol¯1 =
2.992 x 10¯23 gramsNote that the final answer
has been rounded to four significant figures
(from 2.9915 - note use of rounding with five
rule) and that the unit of mole cancels.
Practice #2
• calculate the mass (in grams) of one molecule
of CH3COOH
• The molar mass of CH3COOH is 60.06 g/mol.
The solution is:
• 60.06 g/mol ÷ 6.022 x 1023 mol¯1 = 9.973 x
10¯23 g
Acknowledgements
• Thanks to the ChemTeam for much of the
valuable information on this power point.