Stoichiometry II
Dr. Ron Rusay
Spring 2008
© Copyright 2008 R.J. Rusay
Chemical Equations /
Stoichiometric Calculations
•Example:
C8H18(l)+ O2(g) -----> CO2(g) + H2O(l)
•For the combustion of octane which produces
carbon dioxide and water.
•The equation must conserve mass ,i.e. account
for the mass in product and reactant by having a “balance”
which is an equal number of individual atoms in
reactant and product.
© Copyright 1995-2002 R.J. Rusay
Chemical Equations
1

The equation is “balanced” as before by placing
stoichiometric factors before each of the
molecules in the reaction so that the atoms equal
? C8H18(l) + ? O2(g) -----> ? CO2(g) +
? H2O(l)
1
25/2
8
9
Remove fraction:
2 C8H18(l)+ 25 O2(g)----->
16 CO2(g)+18 H2O(l)
© Copyright 1995-2002 R.J. Rusay
Mass Calculations
1

The Balanced Equation is a balance on a molar
basis which can be related to mass.
2 C8H18(l)+ 25 O2(g)----->
16 CO2(g)+18 H2O(l)
228 g of octane (2 moles)* will react with 800 g of
oxygen (25 moles) to produce 704 g of carbon
dioxide and 324 g of water.
*(2 moles octane x 114 g/mol = 228 g )
© Copyright 1995-2002 R.J. Rusay
Mass Calculations:
Products
Reactants
1.
2.
3.
4.
5.
Balance the chemical equation.
Convert mass of reactant or product
to moles.
Identify mole ratios in balanced equation:
They serve as the “Gatekeeper”.
Calculate moles of desired product or
reactant.
Convert moles to grams.
© Copyright 1995-2002 R.J. Rusay
Mass Calculations:
Products
Reactants
Chemically Relate:
Something (S)
Another Thing (AT)
Mass (S)
Mass (AT)
grams (S)
grams (AT)
© Copyright 1995-2002 R.J. Rusay
Mass Calculations:
Products
Reactants
grams (S)
grams (S)
1 mol (S)
grams (AT)
Avogadro's Number
Atoms
Molecules
Stoichiometry
grams (AT)
(Molecular
Weight)
?
grams (AT)
grams (S)
(Molecular
Weight)
© Copyright 1995-2008 R.J. Rusay
?
"Gatekeeper"
1 mol (AT)
Mass Calculations:
Reactants
Products
�
�
How many grams of salicylic acid are needed to
produce 1.80 kg of aspirin?
Balanced Equation:
Salicylic Acid
Aspirin
COOH
H
COOH
OH
H
+
H
OCCH3
+
CH3COCl
H
H
H
C7H6O3
MW = 138.12
O
HCl
H
H
C2H3OCl
MW = 78.49
C9H8O4
MW = 180.15
HCl
MW= 36.45
Mass Calculations:
Reactants
Products
grams (Aspirin)
grams (Salicylic Acid)
Avogadro's Number
Atoms
Molecules
Stoichiometry
grams (SA)
1800 grams (A)
1 mol (A)
1 mol SA
(Molecular
Weight SA)
? (SA)
grams (A)
1 mol A
(Molecular
Weight A)
"Gatekeeper"
1 mol (SA)
Calculation: http://ep.llnl.gov/msds/Stoichiometry/Aspirin-Calc.html
© Copyright 1995-2000 R.J. Rusay
Mass Calculations:
How many grams of salicylic
acid are needed to produce 1.80 kg of aspirin?
� Balanced
Salicylic Acid
Equation:
Aspirin
COOH
H
COOH
OH
H
+
H
H
C7H6O3
MW = 138.12
OCCH3
+
CH3COCl
H
H
1
O
HCl
H
H
C2H3OCl
MW = 78.49
1 C9H8O4
MW = 180.15
HCl
MW= 36.45
?g C7 = 1.80 x 103 gC9 x [molC9 /180.15gC9 ] x [1 molC7/ 1 molC9] x 138.12 gC7/molC7
?g C7 = 1380 grams
Limiting Reagent
An Ice Cream Sundae
??????
Limiting Reagent
An Ice Cream Sundae
What’s left?
What’s totally consumed?
Limiting Reagent
QuickTime™ and a
Cinepak Codec by Radius decompressor
are needed to see this picture.
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
QuickTime™ and a
Animation decompressor
are needed to see this picture.
QUESTION
The limiting reactant in a reaction:
1) is the reactant for which there is the least
amount in grams.
2) is the reactant which has the lowest
coefficient in a balanced equation.
3) is the reactant for which there is the most
amount in grams.
4) is the reactant for which there is the fewest
number of moles.
5) none of these
ANSWER
5)
none of these
Section 3.10 : Calculations Involving a Limiting
Reactant (p. 106)
The limiting reactant in a reaction is the reactant
that produces the least number of grams of any
product.
Mass Applications: Limiting Reagent
1

How do masses of reactants relate? Is there
enough mass of each reactant for the reaction to
consume all of both of them or will there be some
left of one of them?
2 C8H18(l)+ 25 O2(g)----->
16 CO2(g)+18 H2O(l)
What would happen if only 600. g of O2 were
available for the reaction of 228 g of octane?
© Copyright 1995-2002 R.J. Rusay
Mass Applications:
Determining a Limiting Reagent
�
Does one of the reactants have fewer
stoichiometrically adjusted moles than the
other reactant? If so, the reactant with the
smaller value is the limiting reagent.
Calculation:
� Divide the mass of each reactant by its
respective Molar Mass and by its
Stoichiometric factor from the balanced
equation; then compare the results.
Limiting Reagent Calculation
1

The reactant present in the smallest molar
amount considering stoichiometry limits the mass
basis of any reaction.
2 C8H18(l)+ 25 O2(g)----->
16 CO2(g)+18 H2O(l)
228 g octane / 114 g/mol = 2 mol octane
600. g oxygen / 32 g/mol = 18.75 mol oxygen
2 mol octane / 2 mol (stoich.) = 1
18.75 mole oxygen / 25 mol (stoich.) = 0.75
© Copyright 1995-2002 R.J. Rusay
Mass Effects of the
Limiting Reagent
1
What amount of octane remains unreacted in
the reaction of 600. g of O2 with 228 g of
octane?
600. g O2 x mol O2 /32g O2 x [2mol C8H18
/25mol O2] x 114 g / mol C8H18 = 171 g
C8H18 are reacted
 228 g - 171 g = 57 g C8H18 remain
unreacted

© Copyright 1995-2002 R.J. Rusay
Limiting Reagent /
Theoretical Yield
1
The limiting reagent governs the theoretical
yield of products. For the reaction of 228 g
of octane with 600. g of oxygen, what is
the theoretical yield of carbon dioxide?
2 C8H18(l)+ 25 O2(g)----->
16 CO2(g)+18 H2O(l)
600. g O2 x mol O2 /32g O2 x 16mol CO2 /25mol O2
x 44g / mol CO2 = 528 g CO2
© Copyright 1995-2002 R.J. Rusay
Thoughts to Consider
How much CO2 do you produce per gallon of gasoline
(octane, d= 0.70 g/ml) when gasoline is combusted?
How much CO2 do you personally produce from driving
every week?…. every month? …. every year?
….from other uses and sources?
Why do people in developed nations, like the U.S.,
Japan & in the EU, produce tons more of CO2 per
person than people in under developed or developing
nations?
Does the increase in “man-made” CO2 relate to global
warming?
Greenhouse Gases: The
Chemistry of Warming
What is a greenhouse gas?
The sun’s energy & the molecule’s shape decide.
•Our atmosphere (air) is 78% nitrogen and 21% oxygen.
•Neither are greenhouse gases. They do not absorb infrared radiation (heat).
•However, H2O and CO2 can absorb infrared energy. Without them earth would
be very chilly.
http://zebu.uoregon.edu/1998/es202/l13.html
QUESTION
How many grams of Ca(NO3)2 can be produced
by reacting excess HNO3 with 7.40 g of
Ca(OH)2?
1) 10.2 g
2) 16.4 g
3) 32.8 g
4) 8.22 g
5) 7.40 g
ANSWER
2)
16.4 g
Section 3.9 Stoichiometric Calculations:
Amounts of Reactants and Products (p. 102)
The balanced equation is Ca(OH)2 + 2HNO3 
Ca(NO3)2 + 2H2O. Water is the other product for
an acid/base reaction.
Percent Yield
1 
In synthesis, the actual yield (g) is measured and
compared to the theoretical yield (g). This is the
percent yield:
% = actual / theoretical x 100

If a reaction produced 2.45g of Ibogaine,
C20H26N2O, a natural product with strong promise
in treating heroin addiction, and the theoretical
yield was 3.05g, what is the % yield?
% yield = 2.45g / 3.05g x 100
= 80.3%
© Copyright 1995-2002 R.J. Rusay
QUESTION
The amount of tungsten metal, used to make many lightbulb
filaments, starting with 25.0 grams of WO3 in the following reaction
was 18.0 grams. What percent yield does this represent?
WO3 + 3 H2  W + 3 H2O
1.
2.
3.
4.
25.8%
110%
90.8%
I think I may have confused one of the steps in this calculation
ANSWER
Choice 3 has properly converted the initial grams of WO3 to moles
and used the mole ratio of the equation to determine the maximum
moles of W that would be produced if all the WO3 reacted
(theoretical yield – 19.8 g). The division of the actual yield by the
theoretical yield  100 produces the answer.
Section 3.8: Stoichiometric Calculations: Amounts of Reactants and
Products
Percent Yield
�
A reaction was conducted that theoretically
would produce 0.0025 moles of quinine,
C20 H24 N2 O2 . The actual amount of
isolated quinine was 780 mg. What is the
percent yield of quinine?
� 324 g/mol x 0.0025 mol = 81g =
810mg(theoretical)
� % Yield = 780 mg/ 810 mg x 100
� % Yield = 96%
Mass Calculations:
How many grams of salicylic
acid are needed to produce 1.80 kg of aspirin if the process
produces an 85.0% yield?
� Balanced
Salicylic Acid
Equation:
Aspirin
COOH
H
COOH
OH
H
+
H
OCCH3
+
CH3COCl
H
H
H
1
O
HCl
H
H
C7H6O3
MW = 138.12
C2H3OCl
MW = 78.49
1 C9H8O4
MW = 180.15
HCl
MW= 36.45
?g C7 = 1.80 x 103 gC9 x [molC9 /180.15gC9 ] x [1 molC7/ 1 molC9] x 138.12 gC7/molC7
1.80 x 103 gC9 /85/100 = 2.12 x 103 gC9
?g C7 = 1380 grams
?g C7 = 2.12 x 103 gC9 x [molC9 /180.15gC9 ] x [1 molC7/ 1 molC9] x 138.12 gC7/molC7
?g C7 = 1624 grams
= 1380 grams /85/100