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STOICHIOMETRY
OISE Senior Chemistry AQ
Concept Presentation – November 9, 2011
Presented by Wendy Louis
Expectations & Key Concepts
Expectations Addressed from the ‘Quantities in Chemical Reactions’ Strand of Grade 11 Chemistry –
University Preparation
D2.5 Calculate the corresponding mass, or quantity in moles or molecules, for any given reactant or
product in a balanced chemical equation as well as for any other reactant or product in the chemical
reaction [AI]
D3.4 Explain the quantitative relationships expressed in a balanced chemical equation, using
appropriate units of measure (e.g., moles, grams, atoms, ions, molecules)
Key concept students will learn:
The coefficients of a chemical equation are the basis for interpreting and predicting quantitative
relationships in chemical reactions (stoichiometry).
By the end of the unit students should be able to:
i) Determine the amount of any substance in a reaction, given the amount of any other substance;
ii) Determine the mass of any substance in a reaction, given the amount or mass of any other
substance.
Background Information
The mole concept is introduced and used in stoichiometric calculations. Students
learn to convert the mass of a substance to the number of particles and the
number of moles of the substance.
The mole concept is the link that relates the macroscopic measurement of mass or
volume with the submicroscopic number of particles.
The following is an overview of the lesson sequences for the unit.
The focus of this presentation will be on Lessons 2- 4. It will be presented in depth
since this lesson represents the expectations to be addressed (D 2.5 and D 3.4)
and concepts to be learned. The lab, Gizmos lab work period, and Peer Activity
‘Developing Flowcharts to Solve Problems’ also address these expectations.
Hook : ‘Hamburger Sandwiches’ Problem
Introduce the ‘Hamburger Sandwiches’ problem as an Investigation for students to solve without teacher assistance. This
problem links their understanding of balanced equations, masses of reactants and products and to the concept of
converting moles to mass in chemical equations.
(Lesson courtesy of Journal of Chemical Education Vol. 80 No. 9 September 2003 - see pdf file attached)
‘Hamburger Sandwiches’ Problem
Activity 1: Understanding Formulas and the Use of Equations
The principal has organized a school meeting and you are charged with ordering lunch from a hamburger chain called
Burger Chem that makes sandwiches using two or three identical slices of bread.
You ordered 100 MacChem sandwiches, but somebody made a mistake and 100 HambChem and 100 CheeseChem were
delivered instead. You need to convert these other types of sandwiches into 100 MacChem before the principal notices
the error.
Q: How many HambChem and CheeseChem do you need to obtain 100 MacChem sandwiches if compositions are:
- HambChem: 2 slices of bread (B) and 2 hamburgers (H)
- CheeseChem: 2 slices of bread and 2 slices of cheese (Ch)
- MacChem: 3 slices of bread, 2 hamburgers, and 1 slice of cheese
Activity 2: Understanding Mass Conservation
Q: What is the mass of 100 MacChem sandwiches? Is there any relationship between the mass of 100 MacChem sandwiches
and the masses of 100 HambChem plus 50 CheeseChem?
Data: mass of each slice of bread (B), 25 g; mass of each hamburger (H), 80 g; mass of each cheese slice (Ch), 15 g.
Student Exploration to Discover How to
Use Molar Masses in Stoichiometric
Problems
Following the ‘Hamburger Sandwiches’ problem, the teacher can allow the students to work in
groups on the following problem to observe their problem solving skills (diagnostic
assessment):
If 3.75 grams of nitrogen gas are used in the following reaction, are 3 x 3.75 grams of hydrogen
gas required for the reaction to occur completely? Why or Why not?
N2 (g) + 3 H2 (g) → 2 NH3 (g)
The teacher can provide the following hints:
Convert the macroscopic information / numbers (large-scale) to the microscopic numbers
(small-scale).
“You need to use the molar mass of a compound (microscopic) to find its actual / real
(macroscopic) mass”
Providing Algorithms to Assist with
Problem Solving
Depending on student success and progress with the previous problem, the teacher can present the
following steps to help students organize their solution process.
Four steps required to solve stoichiometric problems (Algorithm)
1. Write the balanced equation for the reaction.
2. Convert the given mass or volume of a reactant or product to moles.
3. Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to
calculate moles of the desired reactant or product.
4. Convert moles of the desired reactant or product to mass or volume required.
OR
Alternatively students could use the GRASS / GRASP (Given, Required, Analysis, Solution,
Sentence /Phrase) method.
Lesson 2: Using Molar Masses in
Stoichiometric Problems
Topic 1: Predicting the Mass of a Reactant
The students and teacher will now conduct formal ‘scientific’ calculations using the following reaction.
Q: Calculate the mass of the second reactant in the reaction:
N2 (g) + 3 H2 (g) → 2 NH3 (g)
Problem: How many grams of hydrogen gas are required for 3.75 grams of nitrogen gas to react completely?
Solution: Using the Four Steps process:
Step 1: The equation is balanced.
Step 2: Convert the given mass or volume of a reactant or product to moles.
Calculate the number of moles of nitrogen gas in 3.75 grams of nitrogen.
The atomic mass of nitrogen is 14.007 u. There are 2 nitrogen atoms, therefore 28.014 u = 28.014 g / mol.
Set up a ratio to calculate the number of moles in 3.75 g of nitrogen:
28.014 g
1 mol
= 3.75 g
x
x = 0.13 moles of nitrogen
Lesson 2: Using Molar Masses in
Stoichiometric Problems cont’d
Step 3: Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to
calculate moles of the desired reactant or product.
1 mole of N2 (g) reacts with 3 moles of H2 (g)
So the number of moles of hydrogen required = 0.13 x 3 = 0.39 moles
Step 4: Convert moles of the desired reactant or product to mass or volume required.
Molar mass of hydrogen = 2 x 1.008 u = 2.016 u = 2.016 g / mol
Set up a ratio to calculate the mass of hydrogen in 0.39 moles:
2.016 g
1 mol
= x
0.39 mol
X = 0.79 g of hydrogen
Concluding statement: 3.75 g of nitrogen requires 0.79 g of hydrogen to react completely.
Lesson 2: Using Molar Masses in
Stoichiometric Problems cont’d
Topic 2: Predicting the Mass of a Product
The students and teacher will attempt to calculate the mass of the product given questions such as...
N2 (g) + 3 H2 (g) → 2 NH3 (g)
Problem: What mass of ammonia is formed when 3.75 g of nitrogen gas reacts with hydrogen gas?
Solution: Using the Four Steps process:
Step 1: The equation is balanced.
Step 2: Convert the given mass or volume of a reactant or product to moles.
Earlier we calculated that 3.75 grams of nitrogen contain 0.133 moles.
Lesson 2: Using Molar Masses in
Stoichiometric Problems cont’d
Step 3: Use the coefficients in the balanced equation to set up the appropriate mole ratio and multiply to calculate moles
of the desired reactant or product.
Use the balanced equation to set up a molar ratio:
1 mole of N2 (g)
0.133 moles
= 2 moles of NH3 (g)
x
x = 0.266 moles of ammonia produced
Step 4: Convert moles of the desired product to mass required.
Calculate the mass of ammonia in 0.266 moles of NH3 (g).
The atomic mass of NH3 = 14.007 u + 3 (1.008 u) = 17.031 u = 17.031 g / mol
Set up a ratio to convert moles to mass:
17.013 g
1 mol
=
x grams
0.266 moles
x = 4.53 g
Concluding Statement: From 3.75 g of nitrogen, 4.53 g of ammonia are produced.
Student Demonstration: Explaining the
Quantitative Relationships Expressed in Balanced
Equations
To summarize the information learned about calculations and
to assist students make the connection between
macroscopic and microscopic levels of reactions have
them develop a table similar to the following using the
previous reaction or they can use another reaction.
The materials they will have available to demonstrate and
teach their learning to the class about their tables :
- jellybeans of 3 different colours (one for each element; to
represent particles in the reaction
- mini ‘mole’ animal toys;
- scales; and
- atoms from molecular kits.
They can then present their tables to the class to explain the
relationship between number of :
- Moles;
- Grams;
- Atoms;
-Molecules; and
- Particles … in the reaction
N2 (g)
3 H2 (g)
1 mole
3 moles
2 moles
3.75 g
0.79 g
4.53 g
0.13 moles
0.39 moles
0.26 moles
2 nitrogen
atoms
6 hydrogen
atoms
2 nitrogen
atoms &
6 hydrogen
atoms
1 molecule
3 molecules
2 molecules
7.86 x 1022
particles
2.35 x 1023
particles
1.60 x 1023
particles
2 NH3 (g)
Additional Lessons Within the Unit

Lesson 3: Take up homework problems on Using Molar masses in Stoichiometric Problems

Lesson 4: Handouts, Mole Ratio, Review definitions and concepts by preparing Concept Map

Lesson 5: Using GIZMOs© computer program to Solve Stoichiometric Problems

Lesson 6: Peer Activity ‘Developing Flowcharts to Solve Problems’

Lesson 7: Quiz - ‘Definitions in Stoichiometry’ and Calculations

Lesson 7 & 8: Introduce ‘Actual &Theoretical Yield - Limiting Reactant’

Lesson 9: Lab - Reaction between aqueous copper (II) sulfate and iron. (see the File ‘Stoichiometry Lab’).
Students will conduct lab to determine the amount of product produced and the percent yield

Lesson 10: Using Molar Volumes In Stoichiometric Problems


Lesson 11: i) Unit Review (Groups prepare a Concept Attainment Chart on ‘Stoichiometry’) and
ii) Start Culminating Task: Stoichiometric Applications in Industry
Student Difficulties & Assessment
Difficulties (see summary handout for potential solutions the teacher can use):
A) Due to the multi-step calculations required to solve mole-to-mass conversion word problems
students may experience difficulty organizing their information.
B) Students believe the coefficients in the balanced equation relate the masses of the
substances.
C) Students believe that changing the coefficient for a substance changes the molar mass.
D) The chemical vocabulary associated with these operations may prevent the students from
applying their mathematical skills to chemical phenomena
Assessment:
Diagnostic Assessment: pre-unit quiz, Hamburger Sandwiches problem, and ammonia reaction
problem
Formative Assessment: Homework, Gizmos work, Concept Attainment Strategy and class
participation, Lab Report results
Summative Assessment: Unit test, quiz, Culminating Task