Chapter 9:
Stoichiometry
Coach Kelsoe
Chemistry
Pages 298–325
Section 9-1:
Introduction to Stoichiometry
Coach Kelsoe
Chemistry
Pages 299–301
Section 9-1 Objectives
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Define stoichiometry
Describe the importance of the mole ratio in
stoichiometric calculations.
Write a mole ratio relating two substances in
a chemical equation
Stoichi-what?
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Stoichiometry (pronounced “sto-key-ometry”)
comes from two Greek words:
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Stoicheion means “element”
Metron means “measure”
Introduction to Stoichiometry
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A lot of our knowledge of chemistry is based
on the careful quantitative analysis of
substances involved in chemical reactions.
Composition stoichiometry deals with the
mass relationships of elements in compounds.
Reaction stoichiometry involves the mass
relationships between reactants and products
in a chemical reaction.
Introduction to Stoichiometry
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The good news: this is not entirely new
material. We’ve already learned about
composition stoichiometry in Section 3-2. We
just never called it that!
Reaction stoichiometry is based on chemical
equations and the law of conservation of
matter.
ALL reaction stoichiometry calculations start
with a balanced equation.
Reaction-Stoichiometry Problems
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The problems in this chapter can be classified
according to the information given in the problem
and the information you are expected to find, called
the unknown.
The given and unknown may both be reactants,
they may both be products, or one may be a
reactant and the other a product.
Masses should be expressed in grams.
Stoichiometric problems are solved by using ratios
from the balanced equation to convert the given
quantity using the following methods.
Reaction-Stoichiometry Problems
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Problem Type 1:
Given and unknown quantities are in moles.
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When you are given the amount of substance in
moles and asked to calculate the amount in moles
of another substance in the chemical reaction, the
general plan is
Amount of given
Amount of unknown
substance (in mol)
substance (in mol)
Reaction-Stoichiometry Problems
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Problem Type 2:
Given is an amount in moles and the
unknown is a mass that is often expressed in
grams.
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When you are given the amount in moles of one
substance and asked to calculate the mass of
another substance in the chemical reaction, the
general plan is
Amount of
given substance
(in mol)
Amount of
unknown substance
(in mol)
Mass of unknown
substance
(in grams)
Reaction-Stoichiometry Problems
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Problem Type 3:
Given is a mass in grams and the unknown is
an amount in moles.
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When you are given the mass of one substance
and asked to calculate the amount in moles of
another substance in the chemical reaction, the
general plan is
Mass of
given substance
(in grams)
Amount of
given substance
(in mol)
Amount of unknown
substance
(in mol)
Reaction-Stoichiometry Problems
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Problem Type 4:
Given is a mass in grams and the unknown is
a mass in grams.
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When you are given the mass of one substance
and asked to calculate the mass of another
substance in a chemical reaction, the general plan
is
Mass of
given
substance
(in grams)
Amount of
Amount of
subtance
(in mol)
substance
(in mol)
given
unknown
Mass of
unknown
substance
(in grams)
Mole Ratio
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Solving any reaction-stoichiometry problem
requires the use of a mole ratio to convert
from moles or grams of one substance in a
reaction to moles or grams of another
substance.
A mole ratio is a conversion factor that
relates the amounts in moles of any two
substances involved in a chemical reaction. It
is obtained directly from the balanced
chemical equation.
Mole Ratio
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Consider the following equation:
2Al2O3(l)  4Al(s) + 3O2(g)
Remember that the coefficients in a chemical
equations satisfy the law of conservation of
matter and represent the relative amounts of
moles of reactants and products.
Therefore, 2 mol of aluminum oxide
decompose to produce 4 mol of aluminum
and 3 mol of oxygen gas. See board for the
six ratios.
Mole Ratio
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For the decomposition of aluminum oxide, the
appropriate mole ratio would be used as a
conversion factor to convert a given amount in
moles of one substance to the corresponding
amount in moles of another substance.
For example, if given 13 mol of aluminum oxide
and need to find out how much oxygen it
decomposes into:
13.0 mol Al2O3 x 3 mol O2 = 19.5 mol O2
2 mol Al2O3
Mole Ratio
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Think about this in terms of making a PB&J
sandwich:
4 slices of bread + 1 cup PB + 1 cup J  2 PB&J sandwiches
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Let’s assume this equation is balanced.
If we have 16 slices of bread, how much PB
will we need?
16 bread x 1 cup PB = 4 cups PB
4 Bread
We can do this for any ingredient!
Mole Ratio
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Mole ratios are exact, so they do not limit the
number of significant figures in a calculation.
The number of significant figures in the
answer is therefore determined only by the
number of significant figures of any measured
quantities in a particular problem.
In layman’s terms, you should have the same
number of sig figs when you multiply by your
mole ratio as you had in your given amount.
Molar Mass
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Remember that the molar mass is the mass,
in grams, of one mole of a substance.
The molar mass is the conversion factor that
relates the mass of a substance to the
amount in moles of that substance.
To solve reaction-stoichiometry problems, you
will need to determine the molar masses
using the periodic table.
Molar Mass
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Returning to the aluminum oxide problem:
2Al2O3(l)  4Al(s) + 3O2(g)
The molar masses can be expressed as so:
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Aluminum
oxide
101.96 g
1 mol Al2O3
OR
1 mol Al2O3
101.96 g
Aluminum
26.98 g
1 mol Al
OR
1 mol Al
26.98 g
Oxygen
32.00 g
1 mol O2
OR
1 mol O2
32.00 g
Molar Mass
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To find the number of grams of aluminum
equivalent to 26.0 mol of aluminum, the
calculation would be as follows:
26.0 mol Al x 26.98 g Al = 701 g Al
1 mol Al
Vocabulary
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Composition stoichiometry
Mole ratio
Reaction stoichiometry