Teacher Background for Stoichiometric Calculations
TEKS 8E (New) Perform stoichiometric calculations, including determination of mass relationships
between reactants and products, calculation of limiting reagents, and percent yield.
TEKS
Chemistry
TEKS 8E
How the TEKS Changed
1998 TEKS
2010–2011 TEKS
This content was not included in the 1998 TEKS.
8E
Perform stoichiometric calculations, including determination of mass
relationships between reactants and products, calculation of limiting
reagents, and percent yield.
Unpacking the TEKS
Explain to students that stoichiometry involves the calculation of quantities in chemical reactions. Calculations using balanced chemical equations are called
stoichiometric calculations.
Use TEKS Lesson 8E: Stoichiometric Calculations to help students understand how to plan and execute calculations involving the amounts of
reactants or products in chemical reactions.
Related Content
TEKS 8E falls under knowledge and skills statement 8. The student can quantify the changes that occur during chemical reactions. Several other 2010–2011
TEKS for Chemistry discuss stoichiometry concepts, including:
8A
Define and use the concept of a mole.
8B
Use the mole concept to calculate the number of atoms, ions, or molecules in a sample of material.
8D
Use the law of conservation of mass to write and balance chemical equations.
9B
Perform stoichiometric calculations, including determination of mass and volume relationships between reactants and products for reactions involving
gases.
1
Content Refresher
Prior Knowledge
A balanced chemical equation tells you
what amounts of reactants to mix and what
amount of a product to expect. Chemists use
balanced chemical equations as a basis for
calculating how much reactant is needed or
how much product will be formed in a
reaction. When you know the quantity of
one substance in a reaction, you can
calculate the quantity of any other substance
consumed or produced in the reaction.
Quantity usually means the amount of a
substance expressed in grams or moles.
However, quantity could just as well be in
liters, tons, or molecules.
Content and Vocabulary
For chemists, stoichiometry is like
bookkeeping—it allows chemists to tally the
amounts of reactants and products using
ratios of moles or representative particles
derived from balanced chemical equations.
Activating Prior Knowledge
Students should know how to balance chemical equations and
understand the mole concept. They should also be familiar with
conversion problems involving mass, moles, and/or volume.
chemical equation: a representation of a chemical reaction;
the formulas of the reactants (on the left) are connected by an
arrow with the formulas of the products (on the right)
conversion factor: a ratio of equivalent measurements used to
convert a quantity from one unit to another
law of conservation of mass: in any physical change or
chemical reaction, mass is conserved; mass can neither be created
nor destroyed
molar mass: a term used to refer to the mass of one mole of any
substance
Engage students in a review of moles and
molar mass. Ask: What is a mole? (A
mole is equivalent to 6.02  1023 particles
of a substance.) Ask: How can you
determine the number of moles of a
substance in a chemical equation? (The
number of moles is represented by the
substance’s coefficient.) Ask: What is
molar mass? (Molar mass is the mass of
one mole of a substance.)
mole: an amount of a substance that contains 6.02  1023
representative particles of that substance
representative particles: the smallest unit into which a
substance can be broken down without a change in composition;
usually atoms, molecules, or ions
Misconception Alert
Students sometimes try to do mass-mass conversions by incorrectly using the mole ratio as a mass ratio. That is, they use grams instead of moles as the units in
the mole ratio and then skip the mass-mole conversion step. Stress that because the number of grams in one mole of a substance varies with its molar mass, a
mass-mole conversion is a necessary intermediate step in mass-mass stoichiometric problems.
Caution students not to make quick judgments when finding the limiting reagent in a reaction. Often, students make a decision based on mass, or they do not
completely finish the calculation and answer prematurely. Encourage students to read a problem slowly, formulate a plan, and write each calculation in steps
until a final answer is obtained. Emphasize the importance of checking the reasonableness of an answer.
2
Covering the TEKS and Their Breakouts
Students will master TEKS 8E when they read and answer:
Lesson Questions 3, 4 where they will (8Ei) perform stoichiometric calculations, including determination of mass relationships between reactants and
products.
Lab Quantitative Analysis Question 1 where they will (8Ei) perform stoichiometric calculations, including determination of mass relationships between
reactants and products.
Lesson Questions 6, 7, 8 where they will (8Eii) perform stoichiometric calculations, including calculation of limiting reagents.
Lesson Questions 9, 10, 11 where they will (8Eiii) perform stoichiometric calculations, including percent yield.
3