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Chapter 12
“Stoichiometry”
Mr. Mole
Stephen L. Cotton
Section 12.1
The Arithmetic of Equations
OBJECTIVES:
• Explain how balanced
equations apply to both
chemistry and everyday
life.
Section 12.1
The Arithmetic of Equations
 OBJECTIVES:
• Interpret balanced chemical
equations in terms of: a) moles,
b) representative particles,
c) mass, and d) gas volume
(Liters) at STP.
Section 12.1
The Arithmetic of Equations
OBJECTIVES:
• Identify the quantities that
are always conserved in
chemical reactions.
STOICHIOMETRY
E.Q.: What mathematical relationships
can be determined from a balanced
chemical equation?
Let’s make some Cookies!
When
baking cookies, a recipe
is usually used, telling the exact
amount of each ingredient.
• If you need more, you can
double or triple the amount
Thus,
a recipe is much like a
balanced equation.
Stoichiometry is…
Greek
for “measuring elements”
Pronounced “stoy kee ahm uh tree”
Defined as: calculations of the
quantities in chemical reactions,
based on a balanced equation.
There are 4 ways to interpret a
balanced chemical equation
#1. In terms of Particles
An Element is made of atoms
A covalent compound (made of
only nonmetals) is made up of
molecules (Don’t forget the diatomic elements)
Ionic Compounds (made of a
metal and nonmetal parts) are
made of formula units
Example: 2H2 + O2 → 2H2O
 Two
molecules of hydrogen and one
molecule of oxygen form two molecules of
dihydrogen monoxide (water).
 Another example: 2Al2O3 Al + 3O2
2 formula units aluminum oxide form 4 atomsAl
and 3 molecules oxygen gas
Now read this: 2Na + 2H2O  2NaOH + H2
#2. In terms of Moles
The
coefficients tell us how
many moles of each substance
2Al2O3 Al + 3O2
2Na + 2H2O  2NaOH + H2
Remember:
A balanced
equation is a Molar Ratio
#3. In terms of Mass
 The
Law of Conservation of Mass applies
 We can check mass by using moles.
2H2 + O2  2H2O
2 moles H2
2.016 g H2
= 4.032 g H2
1 mole H2
+
1 mole O2
31.998 g O2
= 31.998 g O2
1 mole O2
36.04 ggHH2 2++OO2 2
36.030
reactants
In terms of Mass (for products)
2H2 + O2  2H2O
2 moles H2O
18.015 g H2O
= 36.030 g H2O
1 mole H2O
36.030 g H2 + O2 = 36.030 g H2O
36.030 grams reactants = 36.030 grams products
The mass of the reactants must
equal the mass of the products.
#4. In terms of Volume
At
STP, 1 mol of any gas = 22.4 L
2H2
+ O2
 2H2O
(2 x 22.4 L H2) + (1 x 22.4 L O2)  (2 x 22.4 L H2O)
67.2 Liters of reactant ≠ 44.8 Liters of product!
NOTE: mass and atoms are
ALWAYS conserved - however,
molecules, formula units, moles, and
volumes will not necessarily be
conserved!
Practice:
Show
that the following
equation follows the Law of
Conservation of Mass (show
the atoms balance, and the
mass on both sides is equal)
3 2
4 + __O
2 2O3 Al
__Al
Section 12.2
Chemical Calculations
OBJECTIVES:
• Construct “mole ratios” from
balanced chemical
equations, and apply these
ratios in mole-mole
stoichiometric calculations.
Section 12.2
Chemical Calculations
OBJECTIVES:
• Calculate stoichiometric
quantities from balanced
chemical equations using
units of moles, mass,
representative particles, and
volumes of gases at STP.
Mole to Mole conversions
2Al2O3 Al + 3O2
• each time we use 2 moles of Al2O3
we will also make 3 moles of O2
2 moles Al2O3
3 mole O2
or
3 mole O2
2 moles Al2O3
These are the two possible conversion
factors to use in the solution of the problem.
Mole to Mole conversions
How
many moles of O2 are
produced when 3.34 moles of
Al2O3 decompose?
2Al2O3 Al + 3O2
3.34 mol Al2O3
3 mol O2
2 mol Al2O3
= 5.01 mol O2
Conversion factor from balanced equation
If you know the amount of ANY chemical in the reaction,
you can find the amount of ALL the other chemicals!
Steps to Calculate
Stoichiometric Problems
1. Correctly balance the equation.
2. Convert the given amount into
moles.
3. Set up mole ratios.
4. Use mole ratios to calculate moles
of desired chemical.
5. Convert moles back into final unit.
Practice:
2C2H2 + 5 O2  4CO2 + 2 H2O
• If 3.84 moles of C2H2 are burned, how
many moles of O2 are needed?(9.6 mol)
•How many moles of C2H2 are needed to
produce 8.95 mole of H2O? (8.95 mol)
•If 2.47 moles of C2H2 are burned, how
many moles of CO2 are formed? (4.94 mol)
How do you get good at this?
Assignment #1
2 KClO3 ---> 2 KCl + 3 O2
1. How many moles of O2 can be produced by letting 12.00
moles of KClO3 react?
2. How many moles of KClO3 are needed to produce 5.45
moles of KCl?
3. If 10.4 moles of KCl were produced, how many moles of
O2 were also produced?
4. How many moles of KCl can be produced by letting 7.5
moles of KClO3 decompose?
5. How many moles of KClO3 are needed to produce 5.5
moles of KCl?
Mass-Mass Problem:
6.50 grams of aluminum reacts with an excess of
oxygen. How many grams of aluminum oxide are
formed?
4Al + 3O2  2Al2O3
6.50 g Al
1 mol Al
2 mol Al2O3 101.96 g Al2O3
26.98 g Al
4 mol Al
1 mol Al2O3
(6.50 x 1 x 2 x 101.96) ÷ (26.98 x 4 x 1) =
= ? g Al2O3
12.3 g Al2O3
are formed
Another example:
If
10.1 g of Fe are added to a
solution of Copper (II) Sulfate, how
many grams of solid copper would
form?
2_Fe + 3
_CuSO4  Fe2(SO4)3 + 3_Cu
Answer = 17.2 g Cu
Assignment #2
2 K + Cl2 ---> 2 KCl
1. How many grams of KCl are produced from 2.50 g
of K and excess Cl2 ?
2. How many grams of KCl are produced from 1.00
g of Cl2 and excess K?
3. What mass of Cl2 is needed to produce 5.75
grams of KCl?
4. How many grams of K are needed to produce
2.57 grams of KCl?
Other conversion examples….
Mass to particles – Particles to Mass
2Al2O3 Al + 3O2
1) If 2.55 grams of aluminum oxide are available to
decompose, how many atoms of aluminum can be
produced?
2) If 3.15 x 1025 molecules of O2 need to be
produced, how many grams of Al2O3 are needed
for this reaction to take place?
Other conversion examples….
Mass to moles – Moles to Mass
2Al2O3 Al + 3O2
1) If 2.55 grams of aluminum oxide are available to
decompose, how many moles of aluminum can be
produced?
2) If 3.5 moles of O2 need to be produced, how
many grams of Al2O3 are needed for this reaction
to take place?
Assignment #3
Na2O + H2O ---> 2 NaOH
1) How many grams of NaOH is produced from 1.20 x
102 grams of Na2O?
2) How many grams of Na2O are required to produce
1.60 x 102 grams of NaOH?
3) How many grams of Na2O are required to produce
3.25 x 1025 units of NaOH?
4) How many molecules of H2O are needed to
produce 4.75 grams of NaOH?
Volume-Volume Calculations:
 How
many liters of CH4 at STP are
required to completely react with 17.5 L
of O2 ?
CH4 + 2O2  CO2 + 2H2O
1 mol O2 1 mol CH4 22.4 L CH4
17.5 L O2
22.4 L O2 2 mol O2 1 mol CH4
= 8.75 L CH4
Notice anything relating these two steps?
Avogadro told us:
Equal
volumes of gas, at the same
temperature and pressure contain
the same number of particles.
Moles are numbers of particles
You can treat reactions as if they
happen liters at a time, as long as
you keep the temperature and
pressure the same. 1 mole = 22.4 L @ STP
Shortcut for Volume-Volume?
 How
many liters of CH4 at STP are
required to completely react with 17.5 L
of O2?
CH4 + 2O2  CO2 + 2H2O
17.5 L O2
1 L CH4
2 L O2
= 8.75 L CH4
Note: This only works for
Volume-Volume problems.
Section 12.3
Limiting Reagent & Percent Yield
OBJECTIVES:
• Identify the limiting reagent in
a reaction.
Section 12.3
Limiting Reagent & Percent Yield
OBJECTIVES:
• Calculate theoretical yield,
percent yield, and the amount
of excess reagent that remains
unreacted given appropriate
information.
LIMITING & EXCESS
REAGENTS
E.Q.: HOW CAN A BALANCED EQUATION HELP
US DETERMINE THE LIMITING AND EXCESS
REAGENTS IN A CHEMICAL REACTION?
LIMITING & EXCESS
REAGENTS
 WE
WILL DETERMINE LIMITING &
EXCESS REAGENTS THROUGH
CALCULATIONS.
I
WILL DESCRIBE THE STEPS
NEEDED TO CALCULATE THE
LIMITING REAGENT BY WRITING AN
EXIT TICKET.
“Limiting” Reagent
 If
you are given one dozen loaves of
bread, a gallon of mustard, and three
pieces of bologna, how many bologna
sandwiches can you make?
 The limiting reagent is the reactant you
run out of first.
 The excess reagent is the one you have
left over.
 The limiting reagent determines how
much product you can make
Limiting Reagents - Combustion
How do you find out which is limited?
The
chemical that makes the least
amount of product is the
“limiting reagent”.
You can recognize limiting reagent
problems because they will give
you 2 amounts of chemical
Do two stoichiometry problems,
one for each reagent you are given.
 If
10.6 g of copper reacts with
Cugrams
is the of copper
3.83 g sulfur, how many
Limiting
(I) sulfide will be formed?
Reagent,
2Cu + S  Cu2S
since it
1 mol Cu2S 159.16 g Cu2S
1
mol
Cu
produced less
10.6 g Cu
Cu
63.55g Cu 2 mol
1 mol Cu2S
product.
= 13.3 g Cu2S
1
mol
S
3.83 g S
32.06g S
1 mol Cu2S 159.16 g Cu2S
1 mol S
1 mol Cu2S
= 19.0 g Cu2S
Limiting
Limiting
Reactant


10.0g of aluminum reacts with 35.0 grams of chlorine gas to
produce aluminum chloride. Which reactant is limiting, which
is in excess, and how much product is produced?
2 Al + 3 Cl2  2 AlCl3
Start with Al:
10.0 g Al

Reactant: Example 2
1 mol Al
27.0 g Al
Now Cl2:
35.0g Cl2
1 mol Cl2
71.0 g Cl2
2 mol AlCl3 133.5 g AlCl3
2 mol Al
1 mol AlCl3
2 mol AlCl3 133.5 g AlCl3
3 mol Cl2
1 mol AlCl3
= 49.4g AlCl3
= 43.9g AlCl3
Limiting Reagent Problems…..Your
Turn….
1) If 10.3 g of aluminum are reacted with 51.7 g of
CuSO4, which reactant is the limiting reagent?
__Al + __CuSO4 → __Cu + __Al2(SO4)3
2) 15.0 g of sodium oxide reacts with 15.0 g of water.
Calculate which reactant is limiting and how much
product is made.
__Na2O + __H2O ---> __NaOH
Another example:
If 10.3 g of aluminum are
reacted with 51.7 g of CuSO4
how much copper (grams) will
be produced?
2Al + 3CuSO4 → 3Cu + Al2(SO4)3
the CuSO4 is limited, so Cu = 20.6 g
How
much excess reagent will
remain? Excess = 4.47 grams
Limiting & Excess Reagents
Example:
15.0 g of potassium reacts with 15.0 g of iodine. Calculate
how much potassium iodide is produced and how much
excess is left over? 2 K + I2  2 KI
15.0 g K
1 mol K
2 mol KI
39.098 g K 2 mol K
15.0 g I2
1 mol I2
253.808 g I2
2 mol KI
1 mol I2
166.002 g KI
= 63.7 g KI
1 mol KI Excess Reagent
166.002 g KI
1 mol KI
= 19.6 g KI
Limiting Reagent
Finding Excess Practice


15.0 g of potassium reacts with 15.0 g of iodine.
2 K + I2  2 KI
We found that Iodine is the limiting reagent, and 19.6 g
of potassium iodide are produced.
15.0 g I2
1 mol I2
2 mol K
39.098 g K
253.808 g I2
1 mol I2
1 mol K
= 4.62 g K
USED!
15.0 g K – 4.62 g K = 10.38 g K EXCESS
Given
amount of
excess
reagent
Amount of
excess
reagent
actually
used
Note that we started with
the limiting reagent!
The Concept of:
A little different type of yield than
you had in Driver’s Education
class.
What is Yield?
 Yield
is the amount of product made
in a chemical reaction.
 There are three types:
1. Actual yield- what you actually get in
the lab when the chemicals are mixed
2. Theoretical yield- what the balanced
equation tells should be made
3. Percent yield =
Actual
x 100
Theoretical
Example:
6.78
g of copper is produced when
3.92 g of Al are reacted with excess
copper (II) sulfate.
2Al + 3 CuSO4  Al2(SO4)3 + 3Cu
What
is the actual yield?
What
is the theoretical yield? = 13.8 g Cu
What
is the percent yield?
= 6.78 g Cu
= 49.13 %
Details on Yield
Percent
yield tells us how “efficient” a
reaction is.
yield can not be bigger
than 100 %.
Theoretical yield will always be larger
than actual yield!
Percent
• Why? Due to impure reactants; competing
side reactions; loss of product in filtering or
transferring between containers; measuring
Percent Yield
2KClO3 → 2KCl + 3O2
2) If 5.00 g of KClO3 is decomposed to form
potassium chloride and oxygen, what is the percent
yield of the reaction if 1.25 g of O2 are produced?
Ans: 63.78%
Your Turn……
2AlCl3 (s) → 2Al(s) + 3Cl2 (g)
1) If 6.25 g of AlCl3 is decomposed to form aluminum and
chlorine gas, what is the percent yield of the reaction if 2.25
g of Cl2 are produced?
H3PO4 + 3 KOH ---> K3PO4 + 3 H2O
2) If 49.0 g of H3PO4 is reacted with excess KOH,
determine the percent yield of K3PO4 if you isolate 49.0 g
of K3PO4.