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2.C –
Conserving
Matter
2C.1- Keeping Track of Atoms
When a car’s gas empties, where do the
atoms in the gasoline go?
Keeping Track of Atoms
 Law
of Conservation of Matter: Matter is neither
created nor destroyed.
 Since
chemical reactions cannot create or destroy
atoms, chemical equations representing the
reactions must always be BALANCED.
Atomic Perspective:
C
1 Carbon atom
O2
1 oxygen molecule


CO2
1 carbon dioxide molecule
1. What are the reactants in this chemical equation?
2. What are the products in this chemical equation?
3. Are there the same number of atoms on both sides of the equation?
a. Where any atoms destroyed or created?
b. Was the Law of Conservation of Matter maintained?
2 Cu (s) + O2 (g)  2 CuO (s)
COEFFICIENTS - indicates
the number of units of each
substance involved.
1. Does the oxygen molecule have a coefficient?
2. What do the subscripts represent?
3. Can subscripts be removed from chemical equations?
How do we Balance
Equations?
Number of
compounds in
the reaction
Coefficients
2 H2 + O2  2 H2O
Subscripts
# of atoms in a
compound
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation
What do Coefficients Really Mean?
CH4 + 2 O2  CO2 + 2 H2O
H
H
C
H
O
O
O
H
O
O
C
O
H
O
O
H
Total:
1C
4H
4O
The equation is balanced.
H
Total:
1C
4H
4O
H
2 C.2-C.3
-
Accounting for atoms
Nature’s conservation: Balanced chemical
equations
Taken from Kelly Deters
How to Balance By Inspection:
1
Make a table of elements
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
C
H
O
Products
Taken from Kelly Deters
How to Balance By Inspection:
2
Count the number of each element or ion on the
reactants and products side.
Don’t forget to add all the atoms of the same element together—
even if it appears in more than one compound!
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
O
2
3
Taken from Kelly Deters
How to Balance By Inspection:
3
Add coefficients to balance the numbers
Each time you add a coefficient, update your table with the new
quantities of each atom.
_____ CH4 + _____
2 O2 _____
2 H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
Taken from Kelly Deters
How to Balance By Inspection:
4
Place a “1” in any empty coefficient location
Filling each coefficient location lets you and the grader know that you
finished the problem rather than you left some blank because you
weren’t done!
_____
1 CH4 + _____
2 O2 _____
2 H2 O + _____
1 C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
Taken from Kelly Deters
Choosing the Order of Balancing
How do you know what order to balance in?
Start
Elements that appear
only 1 time per side
Save for later
Elements that are
uncombined
Pb + PbO2 + H+  Pb2+ + H2O
Save for later
Elements that appear
more than 1 time per side
Start
Elements in most
complicated molecules
To balance this equation, use the order: O, H, Pb
Taken from Kelly Deters
How is Balancing Affected by
Order?
What happens if we balance in the order
determined in the last slide?
O, H, Pb
_____
1 Pb + _____
1 Pb O2 + _____
4 H+ 
_____
2 H2 O + _____
2 Pb2+
Reactants
O
2
H
1
Pb
2
4
Products
1
2
2
4
1
2
Taken from Kelly Deters
What about a different order?
How is it different if we balance in a different order?
H, O, Pb
_____
1 Pb + _____
1 Pb O2
4
+ _____
2 H+ 
_____
2 H2 O + _____
2 Pb2+
Reactants
2
4
2
1
2
2
1
2
H
1
O
Pb
2
4
Products

You’ll still get to the correct answer, but it will take
longer and be more complicated!
Taken from Kelly Deters
Let’s Practice #1
Example:
Balance the
following
equation
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O
Taken from Kelly Deters
Let’s Practice #1
Example:
Balance the
following
equation
Did you see the “OH” polyatomic ion &
change H2O to HOH?
2 HCl + __
1 Ca(OH)2  __
1 CaCl2 + __
2 H2O
__
HOH
Taken from Kelly Deters
Let’s Practice #2
Example:
Balance the
following
equation
__ H2 + __ O2  __ H2O
Taken from Kelly Deters
Let’s Practice #2
Example:
Balance the
following
equation
2 H2 + __
1 O2  __
2 H 2O
__
Taken from Kelly Deters
Let’s Practice #3
Example:
Balance the
following
equation
__ Fe + __ O2  ___ Fe2O3
Taken from Kelly Deters
Let’s Practice #3
Example:
Balance the
following
equation
4 Fe + __
3 O2  ___
2 Fe2O3
__
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
C.5
The
Mole
Concept
Definition:
Mole – SI unit for counting
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
What is a counting unit?
You’re already familiar with one counting unit…a
“dozen”
A dozen = 12
“Dozen”
12
A dozen doughnuts
12 doughnuts
A dozen books
12 books
A dozen cars
12 cars
A dozen people
12 people
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.
Adopted from "Chemistry
You Need to Know" by
Kelly Deters
A Mole of Particles
Contains 6.02 x 1023 particles
1 mole C = 6.02 x 1023 C atoms
1 mole H2O = 6.02 x 1023 H2O molecules
1 mole NaCl= 6.02 x 1023 NaCl “molecules”
Adopted from "Chemistry
You Need to Know" by
Kelly Deters
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
What does a “mole” count
in?
A mole = 6.02  10 (called Avogadro’s number)
23
6.02  1023 = 602,000,000,000,000,000,000,000
“mole”
6.02  1023
1 mole of doughnuts
6.02  1023 doughnuts
1 mole of atoms
6.02  1023 atoms
1 mole of molecules
6.02  1023 molecules
Adopted from
"Chemistry
You Need to
Know" by
Kelly Deters
C. 6
Molar Mass
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Definition
Molar Mass – The mass for one mole
of an atom or molecule.
Other terms commonly used for the same meaning:
Molecular Weight
Molecular Mass
Formula Weight
Formula Mass
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Mass for 1 mole of atoms
The average atomic mass = grams for 1 mole
Average atomic mass is found on the periodic table
Element
Mass
1 mole of carbon atoms
12.01 g
1 mole of oxygen atoms
16.00 g
1 mole of hydrogen
atoms
1.01 g
Unit for molar mass: g/mole or g/mol
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Molar mass for molecules
The molar mass for a molecule = the
sum of the molar masses of all the
atoms
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Calculating a Molecule’s Mass
To find the molar mass of a molecule:
1
Count the number of each type of atom
2
Find the molar mass of each atom on the periodic
table
3
Multiple the # of atoms  molar mass for each atom
4
Find the sum of all the masses
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass
Example:
Find the
molar
mass for
CaBr2
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass
1
Example:
Find the
molar
mass for
CaBr2
Count the number of each type of atom
Ca
1
Br
2
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass
2
Example:
Find the
molar
mass for
CaBr2
Find the molar mass of each atom on the periodic table
Ca
1
40.08 g/mole
Br
2
79.91 g/mole
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass
3
Example:
Find the
molar
mass for
CaBr2
Multiple the # of atoms  molar mass for each atom
Ca
1  40.08 g/mole =
40.08 g/mole
Br
2  79.91 g/mole =
159.82 g/mole
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass
4
Example:
Find the
molar
mass for
CaBr2
Find the sum of all the masses
Ca
1  40.08 g/mole =
40.08 g/mole
Br
2  79.91 g/mole =
+ 159.82 g/mole
199.90 g/mole
1 mole of CaBr2 molecules would have a mass of
199.90 g
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass &
Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Example: Molar Mass &
Parenthesis
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Sr(NO3)2
Sr
1  87.62 g/mole =
87.62 g/mole
N
2  14.01 g/mole =
28.02 g/mole
O
6  16.00 g/mole =
+ 96.00 g/mole
211.64 g/mole
1 mole of Sr(NO3)2 molecules would have a mass of 211.64 g
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Let’s Practice #2
Example:
Find the
molar
mass for
Al(OH)3
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Let’s Practice #2
Be sure to distribute the subscript outside the
parenthesis to each element inside the parenthesis.
Example:
Find the
molar
mass for
Al(OH)3
Al
1  26.98 g/mole =
26.98 g/mole
O
3  16.00 g/mole =
48.0 g/mole
H
3  1.01 g/mole
=
+ 3.03 g/mole
78.01 g/mole
1 mole of Al(OH)3 molecules would have a mass of 78.01 g
C.8
Molar
Relationships
Adopted from "Chemistry
You Need to Know" by Kelly
Deters
Equations and Molar
Relationships
$1.00 = 4 quarters
If you have 12 quarters than how many $$$’s is
that equal to?
Equations and Molar
Relationships
$1.00 = 4 quarters
If you have 12 quarters than how many $$$’s is that
equal to?
$3.00
What do those coefficients really
mean?
For every 2 moles
of H2…
2 moles of H2O are
produced
2
2
2 H2 + O2  2 H2O
No coefficient = 1
1 mole of O2 is
need to react…
1. What relationships can be found in this equation?
Stoichiometry with Moles
Example:
If 4 moles of H2 reacts
completely with O2, how
many moles of O2 are
needed?
2 H2 + O2  2 H2O
Stoichiometry with Moles
Example:
If 4 moles of H2 reacts
completely with O2, how
many moles of O2 are
needed?
2 H2 + O2  2 H2O
4 mole H2
1
mole O2
2
mole H2
From balanced equation:
2 mole H2  1 mole O2
= ________
2
mole O2
But we can’t measure moles in
lab!
We can’t go to the lab and count or
measure moles…so we need a way to
work in measurable units, such as
grams and liters!
Molecular mass gives the grams = 1 mole of a compound!
Stoichiometry with Moles &
Mass
Example:
How many grams of AgCl will be
precipitated if
0.45 mole AgNO3 is reacted as follows:
2 AgNO3 + CaCl2  2 AgCl + Ca(NO3)2
Stoichiometry with Moles &
Mass
Example:
How many grams of AgCl will be
precipitated if
0.45 mole AgNO3 is reacted as follows:
2 AgNO3 + CaCl2  2 AgCl + Ca(NO3)2
From balanced equation:
2 mole AgNO3  2 mole AgCl
Molar Mass of AgCl:
1 mole AgCl = 143.35 g
0.45 mole AgNO3
2
mole AgCl
2
mole AgNO3
143.35 g AgCl
1
mole AgCl
= ________
65
g AgCl
Stoichiometry with Mass
Example:
How many grams Ba(OH)2 are
precipitated from 14.5 g of NaOH in the
following reaction:
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Stoichiometry with Mass
Example:
How many grams Ba(OH)2 are
precipitated from 14.5 g of NaOH in the
following reaction:
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Molar Mass of NaOH:
1 mole NaCl = 40.00 g
From balanced equation:
2 mole NaOH  1 mole Ba(OH)2
Molar Mass of Ba(OH)2:
1 mole Ba(OH)2 = 171.35 g
14.5 g NaOH
1 mole NaOH
40.00 g NaOH
1
mole Ba(OH)2 171.35 g Ba(OH)2
2
mole NaOH
1
mole Ba(OH)2
31.1
= ________
g Ba(OH)2
Let’s Practice #1
Example:
If you are making 0.57
moles H2O, how many
moles of O2 are needed?
2 H2 + O2  2 H2O
Let’s Practice #1
Example:
If you are making 0.57
moles H2O, how many
moles of O2 are needed?
2 H2 + O2  2 H2O
0.57 mole H2O
1
mole O2
2
mole H2O
From balanced equation:
2 mole H2O  1 mole O2
= ________
0.29 mole O2
Let’s Practice #2
Example:
If you need to precipitate 10.7 g of
Ba(OH)2, how many grams NaOH are
needed?
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Let’s Practice #2
Example:
If you need to precipitate 10.7 g of
Ba(OH)2, how many grams NaOH are
needed?
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Molar Mass of Ba(OH)2:
1 mole Ba(OH)2 = 171.35 g
From balanced equation:
2 mole NaOH  1 mole Ba(OH)2
Molar Mass of NaOH:
1 mole NaCl = 40.00 g
10.7 g Ba(OH)2 1 mole Ba(OH)2
2
mole NaOH
171.35 g Ba(OH)2 1 mole Ba(OH)2
40.00
1
g NaOH
mole NaOH
5.00
= ________
g NaOH
Let’s Practice #3
Example:
How many moles AgNO3 are needed to
react with 10.7 g CaCl2?
2 AgNO3 + CaCl2  2 AgCl + 2
Ca(NO3)2
Let’s Practice #3
Example:
How many moles AgNO3 are needed to
react with 10.7 g CaCl2?
2 AgNO3 + CaCl2  2 AgCl + 2
Ca(NO3)2
From balanced equation:
2 mole AgNO3  1 mole CaCl2
Molar Mass of CaCl2:
1 mole CaCl2 = 110.98 g
10.7 g CaCl2
1
110.98
mole CaCl2
2
mole AgNO3
g CaCl2
1
mole CaCl2
= ______
0.193 mole AgNO3
C.10 Percent
Composition
How many different metals should we use for the coin
design?
 If
using multiple metals, you:
 A)
Need to know specify how much each
material is present
 B) Use: Percentage by Mass of each
material – Percentage Composition
USA - Penny
 During
 After
 Since
1943 - made of zinc coated steel
1943 to 1982 - mostly copper
1982 - made primarily of Zinc
2C.12
Conservation
Depleting Resources
 Renewable
resources
 Nonrenewable resources
 Q. If atoms are always conserved, how
can we say that we may be “running out”
of a resource?
Conserving Resources
 Rethinking
 Reusing
 Replacing
 Recycling
Recycling Aluminum
Dealing with Waste
2C.14 The Life
Cycle of a
Material
Life Cycle of Paper