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Chapter Three:
STOICHIOMETRY
Contents
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Stoichiometry - The study of quantities of materials
consumed and produced in chemical
reactions.
3-1 Counting by Weighing
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3-2 Atomic Masses
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Mass 13C
1.0836129
12
Mass C
Mass of
13
C (1.0836129)(12 amu) 13.003355 amu
Average atomic mass
Exact number by definition
Elements occur in nature as mixtures of isotopes
Carbon = 98.89% 12C; 1.11% 13C; <0.01% 14C 12.01.
Carbon atomic mass = 98.89% of 12 amu 1.11% of 13.0034 amu
(0.9889)(12 amu) (0.0111)(13.0034 amu)
12.01 amu
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Schematic Diagram of a Mass
Spectrometer
Figure 3.1
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Ex 3.1 The Average Mass of an Element
P81
When a sample of natural copper is vaporized and injected
into a mass spectrometer, the results shown in Fig. 3.3 are
obtained. Use these data to compute the average mass of
natural copper. (The mass values for 63Cu and 65Cu are
62.93 amu and 64.93 amu, respectively.)
Solution:
3-3 The Mole
•The number equal to the number of
carbon atoms in exactly 12 grams of
pure 12C.
•1 mole of anything = 6.022 x 1023
units of that thing.
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Ex 3.2 Determining the Mass of a Sample
of Atoms
Americium is an element that does not occur naturally. It
can be made in very small amounts in a device known as a
particle accelerator. Compute the mass in grams of a
sample of americium containing six atoms.
Solution:
Ex 3.4 Calculating Numbers of Atoms
P85
A silicon chip used in an integrated circuit of a
microcomputer has a mass of 5.68 mg. How many silicon
(Si) atoms are present in the chip?
Solution:
Ex 3.5 Calculating the Number
of Moles and Mass
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Cobalt (Co) is a metal that is added to steel to improve its
resistance to corrosion. Calculate both the number of moles
in a sample of cobalt containing 5.00 × 10 20 atoms and the
mass of the sample.
Solution:
3.4 Molar Mass
Molar mass: The mass in grams of one
mole of the compound.
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Ex 3.6 Calculating Molar Mass
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Juglone, a dye known for centuries, is produced from the
husks of black walnuts. It is also a natural herbicide (weed
killer) that kills off competitive plants around the black
walnut tree but does not affect grass and other
noncompetitive plants. The formula for juglone is C10H6O3.
(a) Calculate the molar mass of juglone. (b) A sample of
1.56 x 10-2 g of pure juglone was extracted from black
walnut husks. How many moles of juglone does this
sample represent?
Solution:
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Ex 3.8 Molar Mass and Number of Molecules
Isopentyl acetate (C7H14O2) is the compound responsible
for the scent of bananas. A molecular model of isopentyl
acetate is shown in the margin below. Interestingly, bees
release about 1 μg (1 × 10-6 g) of this compound when
they string. The resulting scent attracts other bees to join
the attack. How many molecules of isopentyl acetate are
released in a typicalbee sting? How many molecules of
isopentyl acetate are released in a typical bee sting? How
many atoms of carbon are present?
Solution:
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Ex 3.9 Calculating Mass Percent
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Carvone is a substance that occurs in two forms
having different arrangements of the atoms but
the same molecular formula (C10H14O) and mass.
One type of carvone gives caraway seeds their
characteristic smell, and the other type is
responsible for the smell of spearmint oil. Compute
the mass percent of each element in carvone.
Solution:
3-5 Percent Composition of
Compounds
Mass percent of an element: For iron in
iron (III) oxide, (Fe2O3)
11169
.
mass % Fe 
100%  69.94%
159.69
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CH3CH2OH
1
t
c
a
Re
Now Try This…
Calculate the number of copper atoms in a
63.55 g sample of copper.
React 2
Consider separate 100.0 gram samples of
each of the following:
H2O, N2O, C3H6O2, CO2
–Rank them from greatest to least
number of oxygen atoms.
3-6 Determining the Formula of a
Compound
Analyzing for Carbon and Hydrogen
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CO2
H2O
%C
%H
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Find the formula
CH5N
C2H10N2
C3H15N3
Formulas
•molecular formula = (empirical formula)n
•[n = integer]
•molecular formula = C6H6 = (CH)6
•empirical formula = CH
Ex 3.11 Determining Empirical
and Molecular Formulas
Determine the empirical and molecular formulas for a
compound that gives the following percentages upon
analysis (in mass percents):
71.65％ Cl, 24.27％ C, 4.07％ H. The molar mass is
known to be 98.96 g/mol.
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Solution:
Dividing each mole value by 2.021 (the smallest
number of moles present), we obtain the
empirical formula ClCH2.
This substance is composed of molecules
with the formula Cl2C2H4.
Ex 3.13 Determining a Molecular Formula
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Caffeine, a stimulant found in coffee, tea , and chocolate,
contains 49.48％ carbon, 5.15％ hydrogen, 28.87％ nitrogen,
and 16.49％ oxygen by mass and has a molar mass of 194.2
g/mol. Determine the molecular formula of caffeine.
Solution:
Rounding the numbers to integers gives the
molecular formula for caffeine: C8H10N4O2.
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Molecular Formula Determination
Method One
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Method 2
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3-7 Chemical Equations
CH 4 O 2  CO2 H 2O
reactants
products
Balancing a chemical equation
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Chemical Equation
A representation of a chemical reaction:
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g)
reactants
products
The Meaning of a Chemical Equation
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3-8 Balancing Chemical Equations
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g)
2 C atoms
2 C atoms
6 H atoms
6 H atoms
7 O atoms
7 O atoms
•The equation is balanced.
•1 mole of ethanol reacts with 3 moles of
oxygen to produce 2 moles of carbon dioxide
and 3 moles of water.
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Balancing Chemical Equations
React 3
Which of the following are true concerning balanced
chemical equations? There may be more than one true
statement.
–The number of molecules is conserved.
–The coefficients tell you how much of each substance
you have.
–Atoms are neither created nor destroyed.
–The coefficients indicate the mass ratios of the
substances used.
–The sum of the coefficients on the reactant side
equals the sum of the coefficients on the product side.
Notice
• The number of atoms of each type of element must be
the same on both sides of a balanced equation.
• Subscripts must not be changed to balance an equation.
• A balanced equation tells us the ratio of the number
of molecules which react and are produced in a
chemical reaction.
• Coefficients can be fractions, although they are
usually given as lowest integer multiples.
• Trial and error is a valid method to balance a chemical
equation.
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Ex 3.14 Balancing a Chemical Equation
P100
Chromium compounds exhibit a variety of bright colors.
When solid ammonium dichromate, (NH4)2Cr2O7, a vivid
orange compound, is ignited, a spectacular reaction occurs,
as shown in the two photographs on the next page.
Although the reaction is actually somewhat more complex,
let’
s assume that the products are solid chromium (III)
oxide, nitrogen gas (consisting of N2 molecules), and water
vapor. Balance the equation for this reaction.
Solution:
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3-9 Stoichiometric Calculations:
Amounts of Reactions and
Products
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Ex 3.16 Chemical Stoichiometry I
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Solid lithium hydroxide is used in apace vehicles to remove
exhaled carbon dioxide from the living environment by
forming solid lithium carbonate and liquid water. What mass
of gaseous carbon dioxide can be absorbed by 1.00 kg of
lithium hydroxide?
Solution:
3-10 Calculations Involving a
Limiting Reagent
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Limiting Reactants
Mixture of CH4 and H2O Molecules
Methane and Water Reacting
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Notice
We cannot simply add the total moles of all
the reactants to decide which reactant
mixture makes the most product. We must
always think about how much product can be
formed by using what we are given, and the
ratio in the balanced equation.
React 4
The limiting reactant is the reactant
a) for which you have the lowest mass in
grams.
b) that has the lowest coefficient in the
balanced equation.
c) that has the lowest molar mass.
d) that is left over after the reaction has
gone to completion.
e) none of the above.
Ex3.18 Limiting Reactant
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Nitrogen gas can be prepared by passing gaseous ammonia
over solid copper (II) oxide at high temperature. The other
products of the reaction are solid copper and water vapor. If
a sample containing 18.1 g of NH3 is reacted with 90.4 g of
CuO, which is the limiting reactant? How many grams of N2
will be formed?
Solution:
Ex 3.19 Calculating Percent Yield
Methanol (CH3OH), also called methyl alcohol, is the
simplest alcohol. It is used as a fuel in cars and is a
potential replacement for gasoline. Methanol can be
manufactured by combination of gaseous carbon
monoxide and hydrogen. Suppose 68.5 kg CO(g) is
reacted with 8.60kg H2(g). Calculate the theoretical yield
of methanol. If 3.57 x 104 g CH3OH is actually produced,
what is the percent yield of methanol?
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Solution:
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Reaction Between Hydrogen and Oxygen