Introduction:
To help students better understand stoichiometry concepts they will be doing a dry lab, an excel
program, use online animation, a wet lab and a tutorial. Included in the module is a pretest to
be taken by the students before the unit is introduced and a post-test for comparison. Before
you begin the unit read over the background information and any terms the students may need
to be introduced to.
Part 1: The smore’s lab will introduce the students to limiting reactants, excess reactants, the
amount of product that is obtained, balancing equations and calculating quantities in grams.
Part 2: The excel program has students use computational tools to see how mass and moles
change as the reaction goes to completion. They will understand how the balanced equation
correlates to the completion of the reactant. The second part of this activity has the students
compute the ratio of the changes in moles of the reactants and products.
Part 3: In this experiment, the students will examine all three reaction conditions, calcium
chloride and sodium phosphate present in the exact mole ratios given in the balanced equation,
calcium chloride present in excess, and sodium phosphate present in excess. The reactants will
be mixed in different mole ratios, in order to attain these three different reaction conditions. The
total number of moles of reactants will be kept the same in each reaction mixture. The actual
yield of one product, calcium phosphate, will be represented by the height of the precipitate
formed in the test tube, and compared with the theoretical yield predicted from the balanced
equation. Upon completion of each reaction, the mixture will be tested for the presence and
identity of any excess reactant.
Part 4: Tutorial
Background and key terms:
Stoichiometry is the study of the quantitative aspects of chemical formulas and reactions.
Knowing the stoichiometry of a formula allows us to relate moles and grams for particular
reactants or products. Knowing the stoichiometry of a reaction allows us to relate amounts of
different substances to each other, and allows us to make quantitative predictions about
chemical reactions. In any balanced chemical equation the coefficients tell how many moles of
each substance are needed for the reaction. Moles combine in the same ratios that molecules
(or atoms) do.
This number of
moles of hydrogen
2 H2
+
To yield this number of moles of water
1 O2
2 H2O
Reacts with this number
of moles of Oxygen
When we are given a reaction between two or more reactants, one may be completely
consumed before the others. The reaction must stop at this point, leaving us with the remaining
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
reactants in excess. The amount of this reactant, then, determines the maximum amount of the
products that can form, and is known as the limiting reactant.
Problem solving process for stoichiometry problems:
Step 1. Write the balanced equation
Step 2. Find the number of moles of the given substance.
Step 3. Inspect the balanced equation to determine the ratio of moles of required substance to
moles of given substance.
Step 4. Express the moles of required substance in terms of grams, then convert moles to
grams..
(start with grams given ) (convert grams to moles ) (use mole ratio ) ( convert moles to grams ) ( end with grams required )
Key terms
Mole – The SI unit for amount of substance. A mole is defined as the amount of a substance that
contains the same number of entities as there are atoms in exactly 12 grams of carbon – 12.
Avagadro’s number - 6.022 x 1023 atoms, molecules or formula units. A conversion factor
(units/mol) between numbers of units (molecules, atoms, or formula units) and moles.
Molar mass- One mole of any substance whose mass is equal to the molecular or formula mass of
the substance in grams. Thus, the molar mass (g/mol) is a conversion factor between numbers of moles
and mass.
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright