STOICHIOMETRY
the study of the
quantitative
aspects of
chemical reactions
(the mathematics
of chemical
reactions)
1
Why Is It Important?
• It allows us to predict how much of a reactant is
consumed or product made in a chemical reaction
• It allows us to determine how much reactant will be
needed to generate a specified amount of product
– Very important if a reactant is expensive; using too much
wastes money
• It allows us to determine how much of an unwanted
product is generated
– Very important to minimize this when we’re dealing with
medicines
– Also important if getting rid of the unwanted product is
expensive
2
Essential Questions
• What is a mole?
• How do we calculate # of moles from # of
atoms and vice versa?
• Why do we need moles?
• What is molar mass?
• How do we calculate the molar mass of an
element or a compound?
3
Let’s relate to something we know
• What is the relationship between the
following words:
– 1 dozen
– 1 century
– 1 decade
• What do you need to figure out the mass of a
dozen eggs if each egg is 10 grams?
Suppose…
we invented a new collection unit called a rapp.
One rapp contains 8 objects.
1. How many paper clips in 1 rapp?
8 paper clips
2. How many oranges in 2.0 rapp?
16 oranges
3. How many rapps contain 40 gummy bears?
5 rapps
5
What is a Mole?
A) A blind furry
animal
B) A brown mark on
your body
C) A double agent
D) An important
chemistry concept
E) All of the above
6
The correct answer is E!
Sorry … we are only going to
focus on the chemistry concept …
7
A Mole Is Just a Number
(a way to count units of something)
Just like …
• A pair = 2
• A dozen = 12
• A baker’s dozen = 13
• A gross = 144
• A ream = 500
… a mole = 602,000,000,000,000,000,000,000
8
Not Just Any Number …
Avogadro’s Number
23
6.02x10
9
Why Avogadro?
• Named after Amedeo Avogadro who studied
quantities of gases and discovered that no
matter what the gas was, there were the same
number of molecules present at similar
conditions
– This is oversimplified
– We will revisit it when we study gases
Definition of Avogadro’s Number
• The number of atoms in exactly 12.00000
grams of the carbon-12 isotope.
(nice to know, but not need to know)
11
Just How Big a Number is a
Mole?
• A mole of M&M’s would cover the continental
United States to a depth of 125 km (~77
miles).
• If the 7 billion people on Earth were to do
nothing but count the gumballs in one mole at
the rate of one gumball per second, it would
take over 2.7 million years to count all the
gumballs.
The Most Important Thing to
Remember
A MOLE IS A NUMBER!
13
The Mole (mol) is an SI Unit
• It is the SI unit for the amount of a substance
• One mole = 6.02 x 1023 particles
–
–
–
–
One mole of eggs = 6.02 x 1023 eggs
One mole of cookies = 6.02 X 1023 cookies
One mole of cars = 6.02 X 1023 cars
One mole of Al atoms = 6.02 X 1023 atoms
• Notice that the NUMBER is always the same,
but the MASS can be very different!
14
Check for understanding
Number of atoms in 0.500 mole of Al
a) 500 Al atoms
b) 6.02 x 1023 Al atoms
c) 3.01 x 1023 Al atoms
Number of moles of S in 1.8 x 1024 S atoms
a) 1.0 mole S atoms
b) 3.0 mole S atoms
c) 1.1 x 1048 mole S atoms
15
Check for Understanding
How many atoms are present in 3.7 mol of
sodium?
2.2 x 1024 Na atoms
How many atoms are present in 155 mol of
arsenic?
9.33 x 1025 As atoms
How many moles of xenon in 5.66 x 1026 atoms?
940. mol Xe
16
A Mole of Particles
Contains 6.02 x 1023 Particles
Regardless of what particles they are
• 1 mol C
• 1 mol H2O
• 1 mol NaCl
=
=
=
6.02 x 1023 C atoms
6.02 x 1023 H2O molecules
6.02 x 1023 NaCl compounds*
*technically, ionics are compounds but not molecules, so
they are called formula units)
*6.02 x 1023 Na+ ions & 6.02 x 1023 Cl– ions
17
Why are Moles Important?
• In chemistry, we don’t work with individual
atoms or molecules or ions because they are
TOO SMALL to be weighed or measured
• To measure these atoms or molecules or ions,
we have to work with LOTS of them
• That’s where the MOLE comes in 6.02X1023 (a
mole) is a practical # of particles to use
because this quantity can be weighed more
conveniently
18
Then and Now … THEN
• When we talked about atoms – we used the
atomic mass unit to measure the mass of an
atom
– It is based on the mass of a carbon-12 atom
• We used the atomic mass from the periodic
table as the weight of an atom of an element
(in amu)
19
Then and Now … NOW
• Recall that the # of atoms in a mole is also
based on a carbon-12 atom
• So, it is reasonable to extrapolate the mass of
one atom (in amu) to the mass of one mole of
atoms (in grams)
20
Molar Mass of an Element
• the mass of 1 mole of an element
• units = grams/mole
• numerically equal to the atomic mass (taken from
the periodic table)
So…the atomic mass from the
periodic table can also be used as
the mass of one mole!
21
Molar Mass Example: Elements
1 mole of C atoms
=
12.0 g
1 mole of Mg atoms
=
24.3 g
1 mole of Cu atoms
=
63.5 g
(Note: these values are taken directly from
the periodic table)
22
Practice
Find the molar mass of the following:
2 moles of Na atoms
=
45.98 g
3 mole of Sn atoms
=
356.13 g
4 moles of He atoms
=
16.0 g
23
Molar Mass of a Compound
(Ionic or Covalent)
• the mass of 1 mole of a compound
• units = grams/mole
• numerically equal to the sum of the atomic
masses (taken from the periodic table)
24
Molar Mass Example: Compounds
1 mole of CaCl2 compound:
1 mole of Ca2+ ions
2 moles of Cl- ions
=
=
40.1 g
2*35.5 g
TOTAL = 40.1 + 71.0 = 111.1 g
(Note: these values are taken directly from the
periodic table)
25
Check for Understanding
What is the molar mass of N2O4?
What is the molar mass of K2O?
What is the molar mass of Al(OH)3?
What is the molar mass of (NH4)2SO4?
26
Answers
N2O4 = 92.0 grams/mole
1 mole of N2O4 = 92g = 6.02x1023 particles
K2O = 94.2 grams/mole
1 mole of K2O = 94.2g = 6.02x1023 particles
Al(OH)3 = 78.0 grams/mole
1 mole of Al(OH)3 = 78.0g = 6.02x1023 particles
27
Mole Jokes!
•
•
•
•
•
•
•
•
Who is Avogadro’s favorite actor?
Mol Gibson…. Get it? Ha!Ha!
Where did Avogadro go on Saturday?
The shopping mole… Wacka!Wacka!
What did Avogadro have on his pancakes?
Molasses …
Why did Avogadro look forward to the year 2000?
It was the start of the new molennium …
28
29
Stop
Work with a partner and practice calculating
moles and molar mass on:
1) Try these problems
2) Worksheet: The Mole and Avogadro's
number
3) Worksheet 11-2
Why do we use moles?
• We know that lab balances don’t directly
weigh moles
– Balances weigh in grams
• We also know that lab balances can’t weigh
individual atoms/molecules/ions – or even
hundreds or thousands of them
– These particles are too small to be weighed on
balances
31
Why do we use moles?
• The mole is a link between the microscopic
(atoms and molecules) and the macroscopic
(grams).
• We will use the concept of the mole to be able
to move between grams and number of atoms
or molecules!
Conversions!
Everything must go through
Moles!!!
molar mass
Grams
Avogadro’s number
Moles
particles
33
Example
moles → grams
Aluminum is often used for the
structure of light-weight bicycle frames.
How many grams of Al are in 3.00 moles
of Al?
Use molar mass to convert moles to
grams
34
Example
grams → moles
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?
Use molar mass to convert grams to moles
35
Example
grams → atoms
How many atoms of K are present in
78.4 g of K?
Convert grams to moles
Then convert moles to atoms
36
Example
molecules → grams
What is the mass (in grams) of 1.20 X
1024 molecules of glucose (C6H12O6)?
Convert molecules to moles
Then convert moles to grams
37
Stop
Work with a partner to complete:
1) Try these on your own
2) Moles and Mass
3) Mixed Mole Problems
HW: ChemQuest 30