Section 3: Stoichiometry: How Much Gas is Produced?
Crucial Chemistry
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Stoichiometry is a term used to describe chemistry computations that theoretically
determine how much product should result from a given amount of reactants.
Stoichiometry calculations can also theoretically determine how much reactant should be
used to get a desired amount of product.
o The calculation process is often referred to as dimensional analysis, which
allows you to “cancel out” your units to solve the stoichiometry problem.
o The “dominoes” or ratios used in this section can be flipped around to help cancel
out the units in dimensional analysis.
When using stoichiometry, it is important to understand what the term mole represents in
chemical reactions.
o A dozen is a counting word used in numerous ways. One dozen “somethings” is
equal to 12 of those “somethings”. For example, there are 12 eggs in one dozen
eggs.
o A mole is a “counting word” used in chemistry. One mole of “somethings” is
equal to 6.022 x 1023 of those “somethings”. For example, there are 6.022 x 1023
atoms in one mole of carbon atoms.
In a balanced chemical equation, the coefficient represents the number of moles needed
to make the reaction work.
o The coefficient is the number written in front of the formulas in a balanced
chemical equation.
o If no number is written, it is understood to equal 1 mole.
o For example, the coefficients in 2KClO3  2KCl + 3O2 are 2, 2, and 3
o This means it takes two moles of KClO3 to produce 2 moles of KCl and 3 moles
of O2.
The molar mass of an element or compound is the mass of one mole of that element or
compound. The units of molar mass are always “grams per one mole”.
o The molar mass of an element is determined by the atomic mass on the periodic
table. For example, one mole of oxygen atoms has a mass of 16.00 grams.
o The molar mass of a compound is equal to the sum of its parts.
 For example, one mole of KClO3 molecules has a mass of 122.55 grams
 1 potassium + 1 chlorine + 3 oxygen = 39.10 + 35.45 + 16.00 + 16.00 +
16.00
o The molar mass can be used as a “domino” to solve stoichiometry problems:
1 mole KClO3
122.55 grams KClO3
or
122.55 grams KClO3
1 mole KClO3
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The molar volume of a gas is the volume of gas that is equal to one mole.
o One mole of any gas (at standard temperature and pressure) is always equal to
22.4 liters of that gas.
o The molar volume can also be used as a “domino” to solve stoichiometry
problems.
1 mole O2
22.4 L O2
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or
22.4 L O2
1 mole O2
Since most reactions don’t produce 100% of the expected products, stoichiometric
calculations can also be used to determine the percent yield of a chemical reaction.
o The percent yield is typically expressed as the (amount found / the amount
expected) x 100%.
According to Staver and Lumpe (1993), "The mole concept, then, is a cornerstone to
building a successful understanding of domain-specific knowledge in chemistry. If
students fail to understand the mole concept, and many do fail, it is likely that their
chemical problem-solving ability will be severely limited" (p. 322).
o Staver, J. R., and Lumpe, A. T (1993). A content analysis of the presentation of
the mole concept in chemistry textbooks. Journal of Research in Science
Teaching, 30, 321-337.