Chemistry
HS/Science
Unit: 06
Lesson: 02
Suggested Duration: 5 days
Empirical and Molecular Formulas
Lesson Synopsis:
This lesson is designed to show how an empirical formula can be determined experimentally using percent composition
and molar mass. The relationship between the empirical and molecular formulas for a substance is also explored.
TEKS:
C.8
C.8C
The student can quantify the changes that occur during chemical reactions. The student is expected to:
Calculate percent composition and empirical and molecular formulas. Supporting Standard
Scientific Process TEKS:
C.2
C.2E
C.2F
C.2G
C.2I
The student uses scientific methods to solve investigative questions. The student is expected to:
Plan and implement investigative procedures, including asking questions, formulating testable hypotheses, and
selecting equipment and technology, including graphing calculators, computers and probes, sufficient scientific
glassware such as beakers, Erlenmeyer flasks, pipettes, graduated cylinders, volumetric flasks, safety goggles, and
burettes, electronic balances, and an adequate supply of consumable chemicals.
Collect data and make measurements with accuracy and precision.
Express and manipulate chemical quantities using scientific conventions and mathematical procedures, including
dimensional analysis, scientific notation and significant figures.
Communicate valid conclusions supported by the data through methods such as lab reports, labeled drawings,
graphs, journals, summaries, oral reports, and technology-based reports.
GETTING READY FOR INSTRUCTION
Performance Indicator(s):
•
Write a laboratory report which includes data collected and all calculations necessary to
determine the empirical formula from the experimental data. Suggest sources of error in the experiment.
Write a paragraph that discusses the difference between empirical and molecular formulas. (C.2G, C.2I;
C.8C)
1E; 5F
Key Understandings and Guiding Questions:
• Empirical formulas give information related to the relative amounts of each element found in a chemical
compound.
— How is the empirical formula determined?
— What information does the empirical formula provide?
— How does percent composition fit in?
— How is the molecular formula determined?
— What information does the molecular formula provide?
Vocabulary of Instruction:
•
•
empirical formula
•
molecular formula
•
percent composition
molar mass
Materials:
Refer to Notes for Teacher section for materials.
©2012, TESCCC
05/07/13
page 1 of 7
Chemistry
HS/Science
Unit: 06 Lesson: 02
Attachments:
1.
2.
3.
4.
5.
6.
7.
8.
Handout: Percent Composition (1 per student)
Teacher Resource: Percent Composition KEY
Handout: What is the Formula for a Hydrate? (1 per group)
Handout: Overboard! PI (1 per student)
Teacher Resource: Overboard! PI KEY
Teacher Resource: Compound Analysis Strips PI (see Adv. Prep., 1 set of data per pair)
Teacher Resource: Performance Indicator Instructions KEY
Advance Preparation:
1. Prior to Day 1, prepare bags of beans for the Explore activity:
• Place 3–6 of each of three types of beans (pinto, black, and kidney recommended) in
resealable, plastic bags.
• Make enough bags for a class set with one bag per 2–3 students.
•
•
2.
•
Each bag of beans will then have a different composition (pinto: black: kidney).
Number and label the bags, and keep a record of how many of each type of bean are in each
bag.
Prior to Day 4 for the Elaborate activity”
Prepare a variety of samples (2 to 5 grams) of the copper (II) sulfate pentahydrate in stoppered
heat-resistant test tubes.
Review all safety and disposal procedures.
•
• Conduct a practice lab, especially if you have to use hot plates instead of Bunsen burners.
3. Prior to Day 4:
• Print enough copies of the Teacher Resource: Compound Analysis Strips PI to have one set of
4.
data per each pair of students.
• Cut out strips of the different compound analysis data provided. The strips are lettered, so you will
need to record which group received which strip.
Prepare attachment(s) as necessary.
Background Information:
During the previous lesson, students developed an understanding of Avogadro’s number and how to apply it to
atoms, formula units, and molecules. The concept of the molar mass of substances was introduced. Avogadro’s
number and the molar mass of a substance were used in chemical calculations. During this lesson, students will
learn to use percent composition and molar mass to calculate an empirical formula. After this unit, students will
use the mole concept as they are introduced to equations, reactions, and stoichiometry in upcoming units.
The formula for a substance can be determined from experimental data using percent composition and molar
mass. The simplest formula is the empirical formula and shows the smallest whole number ratio of atoms in a
compound. The molecular formula can be determined using the empirical formula and the molar mass of the
substance. The molecular formula may be the same as the empirical formula, or it may be a whole number
multiple of the empirical formula.
GETTING READY FOR INSTRUCTION SUPPLEMENTAL PLANNING DOCUMENT
Instructors are encouraged to supplement and substitute resources, materials, and activities to differentiate instruction to address the needs
of learners. The Exemplar Lessons are one approach to teaching and reaching the Performance Indicators and Specificity in the
Instructional Focus Document for this unit. Instructors are encouraged to create original lessons using the Content Creator in the Tools
Tab located at the top of the page. All originally authored lessons can be saved in the “My CSCOPE” Tab within the “My Content” area.
©2012, TESCCC
05/07/13
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Chemistry
HS/Science
Unit: 06 Lesson: 02
INSTRUCTIONAL PROCEDURES
Instructional Procedures
Notes for Teacher
ENGAGE – Class Demographics: A Model of Percent Composition
NOTE: 1 Day = 50 minutes
Suggested Day 1
1. Ask students to organize themselves into groups of three to six students
around the room, mixed male and female.
2. Instruct students to pretend that each group is a molecule, made up of two
kinds of atoms, male and female.
3. Then, ask what the “molecular formula” of their group would be, using F to
represent females and M to represent males (in other words, each group’s
FXMY).
Materials:
• calculator (1 per group)
Instructional Note:
You will be modeling molecular percent
composition by weight, with students
first, and then in the next activity with
bags of beans.
4. Ask students to return to their seats and record their group “molecule” in
their notebooks.
5. Facilitate a discussion using the following questions with each group:
•
What is the percentage of females in your “molecule?”
Work through the calculation as needed with each group.
•
What is the percentage of males in your “molecule”?
Again, work through the calculation as needed.
Check for Understanding:
Accept all reasonable answers to
questions at this point in the lesson, but
make note of misconceptions to address
later in the lesson.
6. Ask students to assume that, like real atoms, all female and male “atoms”
in their group have the same weight—125 lb for female atoms and 160 lb
for male atoms.
Continue the discussion:
•
What is the percent composition of females in your
“molecule” by weight? Work through the calculation as needed.
•
What is the percent composition of males in your
“molecule” by weight? Work through the calculation as needed.
Science Notebooks:
Students record percent composition
model information, questions, and
answers in their notebooks.
7. Ask the members in each group to discuss and then to answer the
following questions in their notebooks:
• If you wanted to find out the formula for a real molecule, how
would you use information on the percent composition of each
kind of atom?
• What do you have to know to find out?
8. Ask for groups to share their thoughts, and accept all reasonable answers.
If students struggle, tell them that you will be revisiting these questions
later in the lesson.
EXPLORE – Bean Bags: Another Model of Percent Composition
1. Divide the class into lab groups of two to three.
2. Distribute a numbered resealable, plastic bag containing three
different types of beans (see Advance Preparation) to each group. Make
©2012, TESCCC
05/07/13
Suggested Days 1 (continued) and 2
Materials:
page 3 of 7
Chemistry
HS/Science
Unit: 06 Lesson: 02
Instructional Procedures
Notes for Teacher
• bags of beans (see Advance
note of which group gets which number bag.
3. Instruct each group to discuss a procedure they will use to find out
what percent by mass of the total bag of beans is represented by each
type of bean.
•
4. Instruct students to write their procedure in their science notebooks.
•
Remind them to include safety precautions as necessary. Remind students
they will be recording data and calculations necessary to determine
percent by mass of each type of bean.
5. Inform students they will need to have their procedures approved
before implementing.
Preparation, class set, 1 bag per
group)
electronic or triple beam balance (1
per group)
calculator (1 per group)
Check for Understanding:
Use class discussion to ascertain
student understanding.
6. Monitor progress and note any procedural problems for later
discussion.
7. When groups have finished, facilitate a discussion and debrief both the
process and student results.
Science Notebooks:
Students record total mass and masses
of each type of beans in their science
notebooks. Additionally, students show
calculations of percent by mass of each
type of bean.
EXPLAIN – Bean Bags: Molecular and Empirical
Suggested Day 2
1. Write the terms molecular and empirical on the board. Explain molecular Attachments:
(actual) and empirical (simplest) formulas.
• Handout: Percent Composition (1
2. Present the molecular formula for fructose, fruit sugar, as C6H12O6, and its
respective empirical formula, CH6O.
•
per student)
Teacher Resource: Percent
Composition KEY
3. Instruct students to write their own definitions of these terms in their
notebooks.
4. Facilitate a discussion using the following questions:
• What information does the empirical formula provide? The ratio of
•
each type of atom in a molecule.
What information does the molecular formula provide? The actual
number of each type of atom in a molecule. The numbers are a whole
multiple of the numbers in the empirical formula.
5. Ask students to refer to their group’s bag of beans. Instruct students to
write a molecular and empirical formula representation of their bean bag
molecule. Suggest letters for each type of bean you used, such as P, B,
and K.
Check for Understanding:
Use the student discussion and handout
as formative assessments of student
understanding.
Science Notebooks:
Students solve percent composition
problems in their science notebooks.
6. Continue the discussion in which students reflect on how they could
determine their bean bag molecule formulas using their mass data. Use
examples.
©2012, TESCCC
05/07/13
page 4 of 7
Chemistry
HS/Science
Unit: 06 Lesson: 02
Instructional Procedures
7. Again, continue the discussion:
• How does percent composition fit in? Each percent composition
•
•
Notes for Teacher
divided by its respective atomic mass yields relative moles.
Comparing moles will yield whole number ratios.
How is the empirical formula determined? The whole number ratios
become the subscripts in the empirical formula.
How is the molecular formula determined? The molecular mass
must be found. Then the molecular mass divided by the empirical
formula mass gives the multiplier for the empirical formula to become
the molecular formula.
8. Instruct students to revisit their notes from the Engage activity and revise
them as necessary based on today’s discussion. Ask several students to
share out with the class. Correct misconceptions as necessary.
9. Assign the Handout: Percent Composition. Instruct students to work
through the problems in pairs and then check each other’s work.
10. Monitor the class, and clarify any misconceptions as necessary.
ELABORATE – Formula of a Hydrate
1. Revisit percent composition, using the examples from the Engage and
Suggested Days 2 (continued) and 3
Explore activities as needed.
2. Divide the class into groups of two to four. Distribute copies of the
Handout: What is the Formula of a Hydrate? to each group. Ask
students to read the handout.
3. Discuss safety precautions (see Safety Notes) and disposal procedures;
Materials:
• safety goggles (1 per student
•
•
answer any questions students may have regarding the instructions.
4. Demonstrate the proper technique for heating a test tube safely.
6. Monitor students as they complete the activity.
•
•
•
•
7. When students have completed their work, collect and tabulate the results.
•
8. Work as a class to determine the average percent composition and
•
5. Instruct students to complete the activity and record data, observations,
and calculations in their science notebooks.
resulting formula(s) for the hydrate.
9. Display the formula for copper (II) sulfate pentahydrate (CuSO 4 ∙ 5 H20).
Ask students to calculate the theoretical percent composition of water in
the hydrate (also known as the accepted value).
aprons (1 per student)
copper (II) sulfate pentahydrate in a
stoppered heat-resistant test tube
(see Advance Preparation, various
samples, 1 per group)
test tube clamp (1 per group)
Bunsen burner (1 per group)
test tube rack (1 per group)
electronic or triple beam balance (1
per group)
MSDS for copper (II) sulfate (1 per
group)
calculator (1 per group)
Attachment:
• Handout: What is the Formula for
a Hydrate? (1 per group)
10. Instruct students to calculate the class average percent error. Direct
students to the STAAR Chemistry Reference Materials for the percent
©2012, TESCCC
05/07/13
Safety Notes:
page 5 of 7
Chemistry
HS/Science
Unit: 06 Lesson: 02
Instructional Procedures
Notes for Teacher
error equation. Note: This resource should be located in their science
notebooks, but provide copies if necessary.
11. Instruct students to write a laboratory report that includes data collected
and all calculations necessary to determine the percent of water in the
copper sulfate hydrate (experimental and theoretical) and the formula.
Instruct students to conclude their reports with a summary of possible
sources of error in the experiment.
12. Use your choice of appropriate laboratory report formats. Clarify
expectations with students.
13. Monitor and assist students as necessary.
All precautions in working with an open
flame should be observed.
Do not wear loose clothing and tie back
long hair.
Wear goggles and aprons.
Do not touch hot objects.
Observe safety procedures for heating a
substance in a test tube.
Demonstrate safe procedures for
lighting and adjusting the burner.
Review the MSDS for use and disposal
of copper (II) sulfate pentahydrate.
Instructional Notes:
Refer to the copper (II) sulfate
pentahydrate as a “hydrated” copper
sulfate.
Although a hot plate may be used
instead of the Bunsen burner in this
activity, the Bunsen burner is the most
efficient source of heat.
Science Notebooks:
Students record all data and
observations in their science notebooks.
EVALUATE – Performance Indicator - Overboard!
Suggested Days 4 and 5
Performance Indicator
•
Write a laboratory report which includes data collected and all
calculations necessary to determine the empirical formula from the
experimental data. Suggest sources of error in the experiment. Write a
paragraph that discusses the difference between empirical and
molecular formulas. (C.2G, C.2I; C.8C)
1E; 5F
1. Refer to the Handout: Overboard! PI, Teacher Resource: Overboard! PI
KEY, Teacher Resource: Compound Analysis Strips PI, and Teacher
Resource: Performance Indicator Instructions KEY for information on
administering the assessment.
©2012, TESCCC
05/07/13
Materials:
• calculator (1 per student)
Attachments:
• Handout: Overboard! PI (1 per
student)
• Teacher Resource: Overboard!
KEY
• Teacher Resource: Compound
Analysis Strips PI (see Adv. Prep.,
page 6 of 7
Chemistry
HS/Science
Unit: 06 Lesson: 02
Instructional Procedures
Notes for Teacher
•
©2012, TESCCC
05/07/13
1 set of data per pair)
Teacher Resource: Performance
Indicator Instructions KEY
page 7 of 7