Chemical Equations /
Stoichiometric Calculations
Stoichiometry II
•Example:
C8H18(l)+ O2(g) -----> CO2(g) + H2O(l)
Dr. Ron Rusay
Fall 2007
•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”
balance”
which is an equal number of individual atoms in
reactant and product.
© Copyright 2007 R.J. Rusay
© Copyright 1995-2002 R.J. Rusay
Chemical Equations
1
The equation is “balanced”
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
(2 moles)* will react with 800 g of
oxygen (25
(25 moles) to produce 704 g of carbon
dioxide and 324 g of water.
*(2
*(2 moles octane x 114 g/mol = 228 g )
© Copyright 1995-2002 R.J. Rusay
1
Mass Calculations:
Products
Reactants
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.
Chemically Relate:
Something (S)
Mass Calculations:
Products
Reactants
grams (S)
1 mol (S)
grams (S)
(Molecular
Weight)
© Copyright 1995-2002 R.J. Rusay
Mass (S)
Mass (AT)
grams (S)
grams (AT)
© Copyright 1995-2002 R.J. Rusay
© Copyright 1995-2002 R.J. Rusay
grams (S)
Another Thing (AT)
Mass Calculations:
Reactants
Products
grams (AT)
Avogadro's Number
Atoms
Molecules
Stoichiometry
grams (AT)
(Molecular
Weight)


Salicylic Acid
?
?
"Gatekeeper"
How many grams of salicylic acid are needed to
produce 1.80 kg of aspirin?
Balanced Equation:
Aspirin
COOH
grams (AT)
H
OH
H
H
COOH
+
1 mol (AT)
OCCH3
H
H
CH3COCl
H
C7H6O3
MW = 138.12
O
H
+
HCl
H
C2H3OCl
MW = 78.49
C9H8O4
MW = 180.15
HCl
MW= 36.45
2
Mass Calculations:
Reactants
Products
grams (Aspirin)
Mass Calculations:
How many grams of salicylic
acid are needed to produce 1.80 kg of aspirin?
grams (Salicylic Acid)
 Balanced
Salicylic Acid
Avogadro's Number
Atoms
Molecules
Stoichiometry
grams (SA)
1800 grams (A)
1 mol (A)
1 mol
SA
1 mol
grams (A)
A
COOH
H
OH
H
H
+
(Molecular
Weight SA)
Calculation: http://ep.
http://ep.llnl
llnl.gov/
.gov/msds
msds//Stoichiometry
Stoichiometry/Aspirin-Calc.html
/Aspirin-Calc.html
1
C7H6O3
MW = 138.12
O
H
OCCH3
H
H
CH3COCl
H
1 mol (SA)
"Gatekeeper"
© Copyright 1995-2000 R.J. Rusay
Aspirin
COOH
? (SA)
(Molecular
Weight A)
Equation:
+
HCl
H
C2H3OCl
MW = 78.49
1 C9H8O4
MW = 180.15
HCl
MW= 36.45
?g C7 = 1.80 x 10 3 gC9 x [mol C9 /180.15g C9 ] x [1 molC7/ 1 molC9 ] x 138.12 gC7 /mol C7
?g C7 = 1380 grams
Limiting Reagent
An Ice Cream Sundae
??????
3
Limiting Reagent
Limiting Reagent
An Ice Cream Sundae
What’s left?
What’s totally consumed?
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
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 O 2 were
available for the reaction of 228 g of octane?
© Copyright 1995-2002 R.J. Rusay
4
Mass Applications:
Limiting Reagent Calculation
Determining a Limiting Reagent
1
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.

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
(stoich.)
18.75 mole oxygen / 25 mol (stoich
.) = 0.75
(stoich.)
© Copyright 1995-2002 R.J. Rusay
Mass Effects of the
Limiting Reagent /
Theoretical Yield
Limiting Reagent
1
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 [2
[2mol C8H18
/25mol
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
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 O 2 x mol O2 /32g O 2 x 16mol
mol O 2
16mol CO2 /25
/25mol
x 44g / mol CO 2 = 528 g CO 2
© Copyright 1995-2002 R.J. Rusay
5
Thoughts to Consider
QUESTION
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?…
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 CO 2 per
person than people in under developed or developing
nations?
Does the increase in “man-made”
man-made” CO2 relate to global
warming?
Percent Yield
1

In synthesis, the actual yield (g) is measured and
compared to the theoretical yield (g). This is the
percent yield:
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
QUESTION
The amount of tungsten metal, used to make many lightbulb
filaments, starting with 25.0 grams of WO 3 in the following reaction
was 18.0 grams. What percent yield does this represent?
WO3 + 3 H2 → W + 3 H 2O
% = actual / theoretical x 100

If a reaction produced 2.45g of Ibogaine,
Ibogaine,
C20 H26 N2O, a natural product with strong promise
in treating heroin addiction, and the theoretical
yield was 3.05g, what is the % yield?
1.
2.
3.
4.
25.8%
110%
90.8%
I think I may have confused one of the steps in this calculation
% yield = 2.45g / 3.05g x 100
= 80.3%
© Copyright 1995-2002 R.J. Rusay
6
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?
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%
 Balanced
Salicylic Acid
Equation:
Aspirin
COOH
H
OH
H
H
COOH
+
OCCH3
H
H
CH3COCl
H
1
O
H
+
HCl
H
C7H6O3
MW = 138.12
C2H3OCl
MW = 78.49
1 C9H8O4
MW = 180.15
HCl
MW= 36.45
?g C7 = 1.80 x 10 3 gC9 x [mol C9 /180.15g C9 ] x [1 molC7/ 1 molC9 ] x 138.12 gC7 /mol C7
1.80 x 10 3 gC9 /85/100 = 2.12 x 10 3 gC9
?g = 1380 grams
C7
?g C7 = 2.12 x 10 3 gC9 x [mol C9 /180.15g C9 ] x [1 molC7 / 1 molC9 ] x 138.12 gC7 /mol C7
?g C7 = 1624 grams
= 1380 grams /85/100
7