 Stoichiometry and The Mole
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The word stoichiometry
derives from two Greek
words: stoicheion (meaning
"element") and metron
(meaning "measure").
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Stoichiometry deals with
calculations about the
masses (sometimes volumes)
of reactants and products
involved in a chemical
reaction.
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It is a very mathematical part
of chemistry, so be prepared
for lots of calculator use.
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What do the terms ream, gross, dozen, pair
have in common?
They are all counting units, designed to make
counting objects easier.
Today, we will look at a counting unit for
chemistry.
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Chemists need a convenient method for
counting
 atoms, molecules, and formula units in a sample
of substance.
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This counting unit is called the Mole.
 What is a Mole????
 It’s Just like a Dozen only bigger.
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SI base unit used to measure the amount of
substance.
It is the number of carbon atoms in exactly 12
grams of Carbon-12.
Has a value of 6.02 x 1023
Called Avogadro’s Number
Named after Amedeo Avogadro
Commonly abbreviated mol.
Just like a dozen only bigger
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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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To appreciate the magnitude of the mole
To practice dimensional analysis
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Molecules for
covalently bonded
substances
 Ex. A water molecule
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Formula Units for Ionic
Substances
 Ex. A formula unit of
sodium chloride
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Atoms for elements
 Ex. An atom of sulfur
 1 mole = 6.02 x 1023
 Used to convert
 Moles to Particles
 Particles to Moles
particles

How many moles of methanol, CH3OH, are
there in 6.53 x 1023 molecules of methanol?
6.02 x 1023 particles
1 mole
or
1 mole
6.02 x 1023 particles
Note that a particle could be an atom OR a molecule!
A sample containing 0.75 moles of CO2 would
contain how many molecules?
How many Fe atoms would be present in 1.27
moles of Fe?
4.47 x 1023 molecules of C6H12O6 would
be how many moles?
Calculate the number of moles contained in
4.50 x 10 24 atoms of zinc.
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How many molecules in 2.8 moles of water?
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Draw a bowl with one dozen grapes in it.
Draw a bowl with a dozen oranges in it.
Compare the masses.
Why are they not the same?
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Draw a circle.
Label it as 1 mole of copper atoms.
Draw another circle.
Label as 1 mole of aluminum atoms.
How many atoms is one mole? Label.
Which weighs more?
Look up masses on periodic table.
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The mass of one mole of a substance is
called "molar mass"
Units g/mol (grams per mole).
Molar mass is the weight in grams of one
mole
One mole contains 6.02 x 1023 entities
Therefore, a molar mass is the mass in
grams of 6.022 x 1023 entities
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Aluminum
Zinc
Copper
Iron
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Just as a dozen oranges would not weigh the
same as a dozen grapes…
A mole of copper atoms does not have the
same mass as a mole of aluminum atoms.
We know that the relative scale for atomic
mass uses the carbon-12 isotope as a
standard.
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Each atom of carbon-12 has a mass of 12
amu.
The atomic mass on the periodic table are
weighted averages of all isotopes.
Since one mole is defined as the number of
carbon-12 atoms in exactly 12 grams of C-12.
Therefore…
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The mass of One Mole of C-12 is 12.0 grams.
Called Molar Mass: the mass in grams of one
mole of a pure substance
 Example:

An atom of Manganese
 Atomic Mass = 54.94 amu
 Molar Mass = 54.94 g/mol
1 mole Ag= 6.02 x 1023 atoms Ag = 107.87 grams Ag
1 mole K = 6.02 x 1023 atoms K = 39.10 grams K
1 mole H2 gas = 6.02 x 1023 molecules H2 = 2.02 g H2
Calculate the mass in grams of 0.0450 moles of
chromium.
How many moles of calcium are contained
in 525 grams of calcium?
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Similar mole-mass, mass-mole conversions
can be made for compounds
We must know the Molar Mass for the
Compound.
How do we calculate Molar Mass of a
Compound?
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Recall that for CCl2F2
The subscripts tell us that one molecule of
freon contains…
 One atom carbon
 Two atoms chlorine
 2 atoms fluorine
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Suppose you have one mole of freon
molecules there would then be
 One mole of carbon atoms
 Two moles chlorine atoms
 Two moles fluorine atoms
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6 moles of CCl2F2 contains
 6 moles carbon atoms
 12 moles chlorine atoms
 12 moles fluorine atoms
Mass in grams of 1 mole equal numerically to
the sum of the atomic masses
1 mole of CaCl2
1 mole Ca x 40.1 g/mol
+ 2 moles Cl x 35.5 g/mol = 70.9 g/mol
= 111.1 g/mol CaCl2
Calculate the molar mass of K2CrO4
Calculate the Molar Mass of K2O
Prozac, C17H18F3NO, is a widely used
antidepressant that inhibits the uptake
of serotonin by the brain. Find its molar
mass.
How many moles are represented by 16.0 g of
ethanol, C2H5OH ?
How many moles of NaCl are in 16.0 grams of
NaCl?
How many moles of potassium hydroxide, KOH
are in 40.6 g?
How many moles of glucose, C6H12O6 are in
27.2 g of glucose?
How many grams in 0.158 moles of KMnO4?
How many grams in 1.2 moles of H2O?
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How many grams in 0.87 moles of H2O2?
How many grams in 0.43 moles of C6H12O6?
X molar mass
Grams
Moles
Moles
Grams
divide by molar mass
The artificial sweetener aspartame
(Nutra-Sweet) formula C14H18N2O5 is
used to sweeten diet foods, coffee and
soft drinks. How many moles of
aspartame are present in 225 g of
aspartame?
6.02 X 1023 particles = 1 mole
AND
1 mole = molar mass (grams)
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You can convert atoms/molecules to
moles and then moles to grams! (Two
step process)
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You can’t go directly from atoms to
grams!!!! You MUST go thru MOLES.
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That’s like asking 2 dozen cookies weigh
how many ounces if 1 cookie weighs 4
oz? You have to convert to dozen first!
How many grams of glucose are in 6.63 x 1023
molecules of glucose, C6H12O6?
Determine the number of molecules found in a
12.4 g sample of H2SO4.
3.14 x 1023 molecules of CO2 are produced in a
chemical reaction. How much would the
sample weigh in grams?
How many atoms are in a 39.8 g sample of Fe?
Volume
at
STP
Representative
Particles
1 mole
22.4 L
1 mole
Mole Road
Map
Everything
must go
through
Moles!!!!
6.02 x 10 23
6.02 x 10 23
1 mole
22.4 L
1 mole
Mole
atomic
mass
1 mole
1 mole
atomic
mass
Mass
A sample of AlCl3 has a mass of 35.6 grams.
How many aluminum ions are present?
How many chloride ions are present?
What is the mass in grams of one formula unit
of aluminum chloride?
How many atoms of Cu are
present in 35.4 g of Cu?
35.4 g Cu
1 mol Cu
63.5 g Cu
6.02 X 1023 atoms Cu
1 mol Cu
= 3.4 X 1023 atoms Cu
How many atoms of K are present in
78.4 g of K?
What is the mass (in grams) of 1.20 X
1024 molecules of glucose (C6H12O6)?
How many atoms of O are present in
78.1 g of oxygen?
78.1 g O2 1 mol O2 6.02 X 1023 molecules O2 2 atoms O
32.0 g O2 1 mol O2
1 molecule O2
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At standard temperature and pressure (STP) a mole
of any gas will occupy 22.4 liters
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Molar Volume @ STP
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1 mole of any gas @STP = 22.4 Liters
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What is STP?
 Standard temperature and Pressure
 101.3 kpa and 273 K
 Arbitrarily chosen conditions that we all agree on so we
know the conditions for measuring the volume
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Convert 427 Liters of CO2 to moles.
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37 liters of O2 to moles.
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How about 3.4 moles of CO to liters?
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122 moles of Methane to liters?
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How many molecules are in 22.4 liters of
methane?
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3.58 x 1023 molecules of propane C8H8 would
occupy how much space at STP?
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You collect 14.2 liters of CO gas from an
experiment. How many molecules would be
in the sample?
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28 grams of H2 are produce from an
experiment. How much volume would they
displace at STP?
Many times a chemist is called upon to
determine the makeup of chemical
compound.
This is the job of an analytical chemist…
How???
Calculating Percent Composition….
% by mass = mass of element x 100
mass of compound
Suppose a 100 gram sample of compound is made up
of 55 grams of X and 45 grams of Y…
55 g X / 100 g compound x 100 = 55 % X
45 g Y / 100 g compound x 100 = 45 % Y
Percents by mass of all element s in a compound must
always add to 100 percent
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The percent composition of a formula is
always the same.
You can assume the sample size is one mole.
This allows the use of molar mass to calculate
percent composition.
What is the percent carbon in C5H8NO4 (the
glutamic acid used to make MSG
monosodium glutamate), a compound
used to flavor foods and tenderize meats?
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Suppose we know the elements in a sample
of a new compound…
We can use this data to determine the
formula of the compound.
 How?
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Formulas give the relative numbers of atoms or
moles of each element in a formula unit - always
a whole number ratio (the law of definite
proportions).
NO2
2 atoms of O for every 1 atom of N
1 mole of NO2 : 2 moles of O atoms to every 1
mole of N atoms
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If we know or can determine the relative number
of moles of each element in a compound, we can
determine a formula for the compound.
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Empirical Formula
The formula of a compound that
expresses the smallest whole number
ratio of the atoms present.
Ionic formula are always empirical
formula
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Molecular Formula
The formula that states the actual
number of each kind of atom found in one
molecule of the compound.
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If the two formulas are different the
molecular formula is a simple multiple of the
empirical formula.
For example:
 Hydrogen Peroxide
▪ Empirical Formula is HO.
▪ Molecular Formula is H2O2
1. Determine the mass in grams of each
element present, if necessary.
2. Calculate the number of moles of each
element.
3. Divide each by the smallest number of moles
to obtain the simplest whole number ratio.
4.
If whole numbers are not obtained* in step 3),
multiply through by the smallest number that
will give all whole numbers
* Be
careful! Do not round off numbers prematurely
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Percent to Mass
Mass to Moles
Divide by small
Multiply ‘til whole
A sample of a brown gas, a major air pollutant,
is found to contain 2.34 g N and 5.34g O.
Determine a formula for this substance.
require mole ratios so convert grams to moles
moles of N = 2.34g of N = 0.167 moles of N
14.01 g/mole
moles of O = 5.34 g = 0.334 moles of O
16.00 g/mole
Formula:
N0.167 O0.334
N 0.167 O 0.334  NO 2
0.167
0.167
A substance has the following composition by
mass: 60.80 % Na ; 28.60 % B ; 10.60 % H
What is the empirical formula of the substance?
Since two compounds can have the
same Empirical Formula we must
determine the actual formula.
 Called Molecular Formula.
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Determine the molar mass of the actual
compound through an experiment.
2. Calculate the molar mass of the empirical
formula.
3. Divide the actual mass by the mass of the
empirical formula.
4. The result shows how many times bigger the
molecular formula is than the empirical formula.
Multiply all subscripts in the empirical formula by
this factor.
1.
A compound has an empirical formula
of NO2. The colorless liquid, used in
rocket engines has a molar mass of
92.0 g/mole. What is the molecular
formula of this substance?
EXAMPLES
Sodium carbonate
decahydrate
Calcium chloride dihydrate
Magnesium sulfate
heptahydrate
Iron (III)phosphate
tetrahydrate
FORMULAS
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A hydrate can be
analyzed by driving off
the water with heat.
The remaining substance
is called the Anhydrous
salt… meaning “without
water”
Some hydrates are a
color different than their
anhydrous salt…
Cobalt (II) chloride
hexahydrate is pink
Without water it is blue.
MgSO4* ?H2O
1.
2.
3.
4.
5.
Heat the sample to drive off all water.
Mass the anhydrous compound.
mass of water = Mass of hydrate – mass of
anhydrous salt
Convert these masses to moles.
Calculate the mole ratio between the compound
and the water molecules.
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Mainly used as dessicants: substances used to
keep things moisture free by absorbing
excess water.