I.
Grade Level/Unit Number:
9-12 Unit 7
II:
Unit Title:
Mole Concept
III.
Unit Length:
7 days (on a 90 min. per day block schedule)
IV.
Major Learning Outcomes:
Students should be able to:
Mole Concept
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V.
Calculate formula mass.
Convert representative particles to moles and moles to representative particles.
(Representative particles are atoms, molecules, formula units, and ions.)
Convert mass of atoms, molecules, and compounds to moles and moles of
atoms, molecules, and compounds to mass.
Convert representative particles to mass and mass to representative particles.
Convert moles to volume and volume to moles at STP.
Calculate molarity given mass of solute and volume of solution.
Calculate mass of solute needed to create a solution of a given molarity and
volume
Solve dilution problems: M1V1 = M2V2.
Calculate empirical formula from mass or percent using experimental data.
Calculate molecular formula from empirical formula given molecular
Calculate molecular formula from empirical formula given molecular weight
Determine percentage composition by mass of a given compound
Perform calculations based on percent composition.
Calculate using hydrates.
Content Objectives Includes (with RBT Tags):
Objective Objective
Number
3.02
Apply the mole concept, Avogadro’s number and
conversion factors to chemical calculations:
 Particles to moles.
 Mass to moles.
 Volume of a gas to moles.
 Molarity of solutions.
 Empirical and molecular formula.
 Percent composition. (dipole/dipole interaction,
dispersion).
 VSEPR theory.
Chemistry- Unit 7
DRAFT
RBT
Tag
C3
1
VI.
English Language Development Objectives (ELD) Included:
NC English Language Proficiency (ELP) Standard 4 (2008) for Limited English
Proficiency Students (LEP)- English Language learners communicate information,
ideas, and concepts necessary for academic success in the content area of science.
Suggestions for modified instruction and scaffolding for LEP students and/or students
who need additional support are embedded in the unit plan and/or are added at the end
of the corresponding section of the lessons. The amount of scaffolding needed will
depend on the level of English proficiency of each LEP student. Therefore, novice level
students will need more support with the language needed to understand and
demonstrate the acquisition of concepts than intermediate or advanced students.
VII.
Materials/Equipment Needed
Activity
How Many Beans are in the Bag?
Materials
Ziplock bags
beans
Ziplock bags
Solid NaHCO3, (NH4)2SO4, KNO3
balances
balances
paper squares
pencils
Counting Formula Units
Making a Mole of Pencil Marks
Percent Composition
Determination of the Empirical Formula
for Zinc Chloride
Water in a Hydrate
Bubble Gum
Popcorn, popped & unpopped
balances
Balances
Hotplates
Weighing boats
Evaporating dishes
6M HCl
Zinc metal
Test tubes
Watch glasses
Balances
Hotplates
Weighing boats
Evaporating dishes
CuSO4 . 5H2O
VIII. Detailed Content Description:
Please see the detailed content description for each objective in the chemistry support
document. The link to this downloadable document is in the Chemistry Standard
Course of Study at:
Chemistry- Unit 7
DRAFT
2
http://www.ncpublicschools.org/curriculum/science/scos/2004/24chemistry
IX.
Unit Notes:
This unit is focused on the concept of the mole, the SI unit for measuring amounts of
substances.
In each unit, Goal 1 objectives which relate to the process of scientific investigation are
included. In each of the units, students will be practicing the processes of science:
observing, hypothesizing, collecting data, analyzing, and concluding. The Goal 1
Objectives are as follows:
COMPETENCY GOAL 1: The learner will develop abilities necessary to do and
understand scientific inquiry.
1.01 Design, conduct and analyze investigations to answer
questions related to chemistry.
 Identify questions and suggest hypotheses.
 Identify variables.
 Use a control when appropriate.
 Select and use appropriate measurement tools.
 Collect and organize data in tables, charts and
graphs.
 Analyze and interpret data.
 Explain observations.
 Make inferences and predictions.
 Explain the relationship between evidence and
explanation.
 Identify how scientists share findings.
Chemistry- Unit 7
DRAFT
This goal and these objectives
are an integral part of each of
the other goals. In order to
measure and investigate
scientific phenomena, students
must be given the opportunity to
design and conduct their own
investigations in a safe
laboratory. The students should
use questions and models to
formulate the relationship
identified in their investigations
and then report and share those
finding with others
Students will be able to:
 Identify questions and
suggest hypotheses.
 Identify variables.
 Use a control when
appropriate.
 Select and use appropriate
measurement tools.
 Collect and organize data in
tables, charts and graphs.
 Analyze and interpret data.
 Explain observations.
 Make inferences and
predictions.
 Use questions and models to
determine the relationships
3
between variables in
investigations.
 Identify how scientists share
findings.
If a teacher follows this curriculum (s)he will have addressed the goals and objectives of
the SCOS. However, teachers may want to substitute other activities that teach the
same concept. The unit length has extra time built in for quizzes, going over
homework, additional practice depending on the nature of the class, and
assessment. Teachers should utilize the textbook as a resource by assigning
homework each day and providing additional guided and independent practice.
Reference Tables:
The North Carolina Chemistry Reference Tables were developed to provide essential
information that should be used on a regular basis by students, therefore eliminating the
need for memorization. It is suggested that a copy be provided to each student on the
first day of instruction. A copy of the reference tables can be downloaded at the
following URL:
http://www.ncpublicschools.org/docs/curriculum/science/scos/2004/chemistry/referencet
ables.pdf
Essential Questions:
Essential questions for this unit are embedded within the unit. Essential questions are
those questions that lead to student understanding. Students should be able to answer
these questions at the end of an activity. Teachers are advised to put these questions
up in a prominent place in the classroom. The questions can be answered in a journal
format as a closure.
Safety: Students should wear chemical splash goggles during any lab activity involving
chemicals. This includes household substances. It is extremely important for the
safety and success of your students that you do ALL activities and labs prior to
assigning them to students. At the beginning of each lab, the teacher should address
any specific safety concerns relating to the activity.
Computer Based Activities:
Several of the recommended activities are computer based and require students to visit
various internet sites and view animations of various biological processes. These
animations require various players and plug-ins which may or may not already be
installed on your computers. Additionally some districts have firewalls that block
Chemistry- Unit 7
DRAFT
4
downloading these types of files. Before assigning these activities to students it is
essential for the teacher to try them on the computers that the students will use and to
consult with the technology or media specialist if there are issues. These animations
also have sound. Teachers may wish to provide headphones if possible.
X.
Global Content: Aligned with 21st Skills
One of the goals of the unit plans is to provide strategies that will enable educators to
develop the 21st Century skills for their students. As much as students need to master
the NCSOS goals and objectives, they need to master the skills that develop problem
solving strategies, as well as the creativity and innovative thinking skills that have
become critical in today’s increasingly interconnected workforce and society. The
Partnership for 21st Century Skills website is provided below for more information about
the skills and resources related to the 21st Century classroom.
http://www.21stcenturyskills.org/index.php?option=com_content&task=view&id=27&Ite
mid=120
NC SCS
Chemistry
1.01 & 3.02
21st Century Skills
Communication Skills
Conveying thought or opinions effectively
Activity
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1.01 & 3.02
When presenting information,
distinguishing between relevant and
irrelevant information

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
Chemistry- Unit 7
DRAFT
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
Formula Mass
Problems
Mole Problems
Percent
Composition
Problems
Empirical &
Molecular Formula
Problems
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
5

1.01 & 3.02
Explaining a concept to others
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
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
1.01 & 3.02
1.01 & 3.02
1.01 & 3.02
1.01 & 3.02
Interviewing others or being interviewed
Computer Knowledge
Using word-processing and database
programs
Developing visual aides for presentations
Using a computer for communication
Learning new software programs
Employability Skills
Assuming responsibility for own learning

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

Chemistry- Unit 7
DRAFT
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
Formula Mass
Problems
Mole Problems
Percent
Composition
Problems
Empirical &
Molecular Formula
Problems
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
Formula Mass
6
1.01 & 3.02
1.01 & 3.02
1.01 & 3.02
3.02
Persisting until job is completed
Working independently
Developing career interest/goals
Responding to criticism or questions
Information-retrieval Skills
Searching for information via the
computer
Searching for print information
Searching for information using
community members
Language Skills - Reading
Following written directions
Problems
 Mole Problems
 Percent
Composition
Problems
 Empirical &
Molecular Formula
Problems
All activities
WIZARD assessment
questions
Most of the activities can
be presented as
opportunities for students
to follow written directions.
The teacher will have to
work with most students to
develop this skill over
time. The following
activities are well suited to
developing skills in
following directions:
 How Many Beans
are in the Bag?
 Counting Formula
Units
 Percent
Composition
 Determination of
the Empirical
Formula for Zinc
Chloride
 Water in a Hydrate
 Formula Mass
Identifying cause and effect relationships
Summarizing main points after reading
Locating and choosing appropriate
Chemistry- Unit 7
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7
1.01 & 3.02
reference materials
Reading for personal learning
Language Skill - Writing
Using language accurately
Organizing and relating ideas when
writing





1.01 & 3.02
1.01 & 3.02
1.01 & 3.02
Proofing and Editing
Synthesizing information from several
sources
Documenting sources
Developing an outline
Writing to persuade or justify a position
Creating memos, letters, other forms of
correspondence
Teamwork
Taking initiative
Working on a team
All activities



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
1.01 & 3.02
Thinking/Problem-Solving Skills
Identifying key problems or questions
Evaluating results



Chemistry- Unit 7
DRAFT
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
8

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


3.02
Developing strategies to address
problems





Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
Formula Mass
Problems
Mole Problems
Percent
Composition
Problems
Empirical &
Molecular Formula
Problems
How Many Beans
are in the Bag?
Counting Formula
Units
Percent
Composition
Determination of
the Empirical
Formula for Zinc
Chloride
Water in a Hydrate
Developing an action plan or timeline
ENGAGE: (30 minutes)
How Many Beans are in the Bag?
This activity allows students to experience “Counting By Weighing” which allows
students to experience how chemists count the number of atoms, molecules, or
particles in a sample of matter. It is an introductory activity to the mole.
Advanced Preparation:
 Determine the mass individual Ziploc bags and write the mass on the bag in
permanent maker.
 Put 50 beans in each bag. Once prepared, these bags can be used from yearto-year.
 Fill a small beaker with beans. Students may use these beans to count, weigh,
etc.
Chemistry- Unit 7
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9
Suggestions:
 Begin by having each student record their answer to the question that begins this
activity.
 Students may work individually or in pairs.
 Students will want to immediately begin to weigh beans without giving much
thought to an experimental procedure. Therefore, you may want to initial
students’ plans before they are allowed in the lab.
 Some students will use just one bean from their beaker, assuming that it will
represent the average mass of all the beans. As students start their
experimental procedure, you will want to circulate throughout the lab. If you see
students using just one bean, you may want to facetiously ask them how they
knew that the one bean on the balance was the “magic” bean. This gives you the
opportunity to help them come to the conclusion that get a good average mass of
the beans, one needs to use more than one and the more the better.
 After students have experimentally determined the number of beans in their bag,
you can let them count how many are truly in their bag.
 Conclude this activity with a class discussion, having students share their
procedure for determining the number of beans in their bag without counting.
Include in this discussion what it means to a chemist to “count by weighing”?

Essential Question:
What does it mean to count by weighing?
Language (ELP) Objectives for LEP Students:
 Explain procedure for this activity verbally or in written form.
 Think-pair –share each step of the activity with lab partner, teacher, or
class.
EXPLORE:
DETERMINING NUMBER WITHOUT COUNTING
Question: How many beans are in the bag? You CANNOT open the bag!
_________________________
Assignment:
 Develop an experimental procedure for determining the number of beans in the bag
without opening it.
 List the materials you will need to carry out your plan.
 Have your plan approved by your teacher.
 Carry out your plan, making notes of any changes made to your original plan.
Conclusion:
1. How many beans are in the bag? Explain how you determine the number.
Chemistry- Unit 7
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10
2. What do chemists meant by the phrase, “counting by weighing”?
ELABORATE: (15 minutes)
Calculating Molecular/Formula Mass


Show students how to find the molar mass of elements and compounds. Include
terminology (molecular mass, formula mass, and molar mass).
Provide guided practice from the textbook or other resource. Students should
be given names of compounds to review writing formulas.
Essential Question:
How would you calculate the molar mass of an element and compound?
Language (ELP) Objectives for LEP Students:
 Construct a paragraph explaining how they would calculate molar
mass using the following key terms: molecular mass, formula
mass and molar mass.
 Chose one of the given compounds and explain how the process
of accurately writing the formula.
EXPLORE: (30 minutes)
Counting Formula Units
In this activity, students will discover that Avogadro’s Constant is equal to a mole. This
activity will lead into mole conversions.
Advanced preparation:
 Determine the mass of 3 Ziplock bags and write the mass on the bag.
 Add a one mole of each compound in the table to an individual bag.
As part of the class discussion, have students fill in the table below as you make mass
measurements.
 Have students record the mass that is written on the baggie containing one of the
compounds.
 Have students record the mass of the compound and the baggies.
 Have students calculate the mass of each compound.
 Tell students that you know how many formula units are in each baggie because
you counted out each and placed it in the baggie. Have students fill in the last
column with Avogadro’s number. Hopefully, students will know that you did not
really count out that many formula units in each baggie.
 Have students calculate the formula mass of NaHCO3, (NH4)2SO4, and KNO3.
Chemistry- Unit 7
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11


Students should recognize that the molar mass is equal to a mole and that each
baggie contains a mole.
At this point, they should understand the relationship between a mole and
Avogadro’s constant.
Essential Question:
What does it mean to count formula units?
Counting Formula Units
MASSES
Compound
Baggie
Baggie +
compound
Compound
Number of
Formula Units
NaHCO3
(NH4)2SO4
KNO3
Predict (calculate) the molar mass of the three compounds in the table above.
1. NaHCO3 ____________________
2. (NH4)SO4 ____________________
3. KNO3
____________________
On what criteria did you you’re your prediction?__________________________________
ELABORATE: (45 minutes)
Mole Conversion
Chemistry- Unit 7
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12
The teacher can use the following notes to guide students through learning mole
conversion calculations. The teacher should provide examples from the text or other
resource of different mole conversions as indicated by the objective:
Particles to moles
o Convert representative particles to moles and moles to representative
particles. (Representative particles are atoms, molecules, formula units,
and ions.)
Mass to moles
o Convert mass of atoms, molecules, and compounds to moles and moles
of atoms, molecules, and compounds to mass.
o Convert representative particles to mass and mass to representative
particles.
Volume of a gas to moles
o Convert moles to volume and volume to moles at STP.
The Mole
Why are M & Ms sold in a package rather than individually?
List other items we package and then refer to as a package rather than the individual
item:
100 pennies =
2 socks =
12 eggs =
A chemist does the same thing with atoms! Why?
A chemist’s package is a mole.
1 mole = 6.02 x 1023 particles.
6.02 x 1023 is called Avogadro’s number.
It has been experimentally determined that when one measures out the mass of an
element equal to its average atomic mass, the number of atoms contained in the
sample is equal to 6.02 x 1023 atoms.
In a “package” 6.02 = 10 23 atoms equals one mole.
6.02 x 1023 atoms = 1 mole = Avogadro’s number
Chemistry- Unit 7
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13
22.4 L/mole of gas
Grams
Volume of Gas (at STP)
Moles
Particles
(atoms,
molecules, etc.)
Avogadro’s #
6.02 x 1023 particles/mole
From periodic table
g/mole
Examples.
1.
2.
3.
4.
5.
How many atoms in 400.0g of sulfur?
What is the mass of 1.2 x 1024 atoms of magnesium?
What is the mass of 2.5 moles of oxygen atoms?
Given 18 grams of water, how many molecules do you have?
What is the mass, in grams, of 1.2 x 1024 formula units of sodium chloride?
Essential Question:
How do you convert particles to moles, mass to moles, and volume of a gas to moles?
Language (ELP) Objectives for LEP Students:
 Construct a flowchart or concept map which demonstrating the different
mole conversions discussed in class
 Construct a visual representation using clay, Styrofoam, card board, etc.
EVALUATE: (45 minutes)
Mole Conversion
This activity will allow students to assess their understanding of mole calculations with
guided and independent practice. After students have completed these questions, the
teacher will evaluate students’ understanding by going over the questions with the
students.
Essential Question:
How do you convert particles to moles, mass to moles, and volume of a gas to moles?
Chemistry- Unit 7
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14
Mole Conversions
Make the following conversions, showing all work.
1) 77.56 g of CaCO3 to mole
2) 2.55 x 1024 molecules of KCl to
grams
3) 0.0931 mol of BaCl2 to grams
4) 1.55 x 1022 molecules to moles
5) 0.664 moles of HF to molecules
units
6) 86 g of Fe(NO3)3 to formula
7) 7.88 x 1023 molecules of CO2 to grams
8) 2.93 mol of MgF2 to grams
9) 7.56 g of MgCO3 to moles
10) 8.55 x 1021 molecules of NaCl
to grams
11) 234.5 mol of ZnF2 to grams
12) 1.55 x 1026 molecules to
moles
13) 9.4 moles of MgS to molecules
14) 5066 g of Al(ClO3)3 to formula
units
15) 9.68 x 1023 molecules of H2O to grams
16) 2.5 x 10-3 mol of CuF2 to
grams
17) 58.4 L CO2 to moles
18) 2.5 moles O2 to L
19) 256 g NO2 to L
20) 5.0 L CH4 to g
Chemistry- Unit 7
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15
EXPLORE: (30 minutes)
Making a Mole of Pencils Marks
Students will make pencil (graphite – Carbon) marks on a piece of paper for 2 minutes.
They will calculate the mass, the atoms, and the number of minutes it would take to
place a mole of carbon on the piece of paper.
Materials
 balances
 paper squares
 pencils
Advanced preparation
 Cut paper into squares (4” x 4” work well)
 sharpen regular graphite pencils (do not use mechanical pencils – students need
to press hard with their pencil to deposit enough graphite to make a measurable
difference in their paper)
Sample Data & Calculations
1. Calculate the mass of graphite (carbon) you placed on your piece of paper in two
minutes.
2. Calculate the number of carbon atoms you placed on your paper in two minutes.
Chemistry- Unit 7
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16
3. Calculate how many minutes it would take you to place a mole of carbon atoms on
your piece of paper.
Essential Question:
How do you determine the mass and number of atoms of carbon that can be marked on
a piece of paper in 2 minutes?
Making a Mole of Pencil Marks
Procedure:
1. Read the entire procedure and make a table to record collected data.
2. Measure and record the mass of a piece of paper.
3. Place as many graphite (carbon) pencils marks as you can on your piece of premassed paper.
4. Measure and record the mass of the paper with the pencil marks.
Calculations: You MUST show ALL you work.
1. Calculate the mass of graphite (carbon) you placed on your piece of paper in two
minutes.
2. Calculate the number of carbon atoms you placed on your paper in two minutes.
3. Calculate how many minutes it would take you to place a mole of carbon atoms on
your piece of paper.
EXPLORE: (45 minutes)
This activity will introduce percent composition to students using bubble gum and
popcorn. Students will calculate the percentage of sugar in bubble gum and the
percentage of water in popcorn. This activity can provide relevance to the curriculum.
The teacher can discuss different types of popcorn and how they pop differently based
on the amount of water in the different brands. It is also important to discuss water
reaching the boiling point causing vaporization, which in turn, causes the kernel to pop.
Chemistry- Unit 7
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17
After the activity, the teacher should provide examples of percent composition with
compounds (include hydrates) and guided practice from the text or other resource.
Essential Question:
How do you find the percentage composition of sugar in bubble gum and water in
popcorn?
Percentage Composition
In this experiment, you will determine the % of sugar in bubble gum and the percentage
of water in popcorn.
BUBBLE GUM
1. You will need one person in your group to volunteer to chew gum (must chew for
15 minutes).
2. Measure the mass of the unchewed gum still in its wrapper (note flavor).
3. Unwrap gum (save wrapper) and chew gum for 5 minutes.
4. Place gum back on wrapper and reweigh.
5. Repeat for two more 5 minute chew cycles, reweighing after each cycle.
6. Calculate the % of mass lost by the gum. This is the % of sugar that was in the
gum.
% sugar = (total mass lost ÷ original mass of gum) x 100
7. Draw a graph of mass of gum vs. time, and explain what the graph shows
8. Compare results involving various flavors of gum.
9. Complete a lab report. Write a conclusion paragraph.
POPCORN
1. Mass 3 different size samples of unpopped and popped corn. Your smallest
sample should be 25 kernels.
2. For each example, calculate % of mass lost by popped corn. This represents the
% of water that was in the corn.
% water = (difference in mass between unpopped and popped
Corn ÷ mass of unpopped corn) x 100
3. Calculate the average % of water in your 3 samples.
4. Complete a lab report. Write a conclusion paragraph (include an explanation of
how popcorn pops and how age may affect the popping process).
LAB REPORT FORMAT:
Title, Purpose, Data Table, Calculations, Graph (bubble gum only), conclusion
Chemistry- Unit 7
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18
Percentage Composition
Part I: Bubble Gum
Mass (unchewed gum in wrapper)
Mass (after 5 minutes)
Mass (after 10 minutes)
Mass (after 15 minutes)
% Composition of sugar:
total mass lost
original mass of gum
x 100 =
Draw a graph of mass of gum vs time and explain what the graph shows.
*****ON BACK!!
Part II: Popcorn
Sample #
Mass (unpopped)
Mass (popped)
% Composition of water: difference in mass between popped and unpopped corn
x100 =
mass of unpopped corn
Sample 1:
Sample 3:
Sample 2:
Average % of water in your 3 samples:
Chemistry- Unit 7
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19
EXPLAIN:
Have students present their answers and calculations from the Percent Composition lab
to the class. Instruct them to explain their reasoning.
ELABORATE: (30 minutes)
The teacher should provide examples of percent composition with compounds (include
hydrates) and guided practice from the text or other resource.
Percent Composition Practice
For problems 1-6, calculate the percent of each element in the compound.
1. NH3
2. KMnO4
3. BaCl2
4. (NH4)2SO4
5. CH3Cl
6. C6H12O6
For problems 7 & 8, calculate the percent of each element in the hydrate. Then
calculate the percentage of water in the hydrate.
7. CuSO4 ● 5H2O
8. MgSO4 ● 7H2O
For problems 9 & 10, calculate the percent water present in each hydrate.
9. SnCl2 ● 2H2O
10. Na2CO3 ● 10H2O
Key:
1. 82.22%N, 17.78%H
2. 24.74%K, 34.76%Mn, 40.50%O
3. 65.95%Ba, 34.05%Cl
4. 22.10%N, 6.11%H, 24.34%S, 48.42%O
5. 23.79%C, 6.00%H, 70.21%Cl
6. 39.99%C, 6.73%H, 53.28%O
7. 25.45%Cu, 12.84%S, 57.66%O, 4.04%H; 36.08%H2O
8. 9.47%Mg, 12.50%S, 68.61%O, 5.51%H; 49.17%H2O
9. 15.97%H2O
10. 62.97%H2O
Essential Question:
How do you determine find the percentage composition of each component in a
compound?
ELABORATE: (30 minutes)
Empirical Formulas
Chemistry- Unit 7
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20
The teacher should introduce empirical formulas & molecular formulas
The teacher can use the following notes to guide students through learning empirical &
molecular formula calculations
Empirical & Molecular Formulas
Empirical Formula – the formula for a compound expressed as the smallest possible
whole-number ratio of subscripts of the elements in the formula.
Molecular Formula- the formula for a compound in which the subscripts give the actual
number of each element in the formulas it truly exists.
Molecular Formula
H2O
Empirical Formula
CH3COOH
CH2O
C6H12O6
Notice two things:
1. The molecular formula and the empirical formula can be identical.
2. You scale up from the empirical formula to the molecular formula by a whole
number factor.
Calculating Empirical Formulas
A Simple Thyme for a Simple Formula by Joel S. Thompson
Percent to mass
Mass to mole
Divide by small
Multiply ‘til whole
Example 1. A compound consists of 72.2% magnesium and 27.8% nitrogen by mass.
What is the empirical formula?
Example 2. A compound is analyzed and found to contain 68.54% carbon, 8.63%
hydrogen, and 22.83% oxygen. The molecular weight of the compound is known to be
approximately 140 g/mol. What is the empirical formula?
Chemistry- Unit 7
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Calculating Molecular Formulas
To determine the molecular formula you must know the empirical formula and the
molecular weight/mass.
Example 3: A 1.50 g sample of hydrocarbon undergoes complete combustion to
produce CO2 and H2O. The empirical formula of this compound is CH3. Its molecular
weight has been determined to be about 78. What is the molecular formula?
Example 4: Caffeine has the following percent composition: carbon 49.48%, hydrogen
5.19%, oxygen 16.48% and nitrogen 28.85%. Its molecular weight is 194.19 g/mol.
What is its molecular formula?
Essential Questions:
What is an empirical formula and how is it calculated?
How would you compare an empirical formula to a molecular formula?
How is a molecular formula calculated?
EVALUATE: (30 minutes)
This activity will allow students to assess their understanding of empirical & molecular
formula calculations with guided and independent practice. After students have
completed these questions, the teacher will evaluate students’ understanding by going
over the questions with the students.
Empirical Formulas Practice
1. 36.5 g Na
25.4 g S
2. 53.73% Fe
46.27% S
3. 63.5% Fe
36.5 % S
Chemistry- Unit 7
DRAFT
38.1 g O
22
4. 63.1% Mn
36.9% S
5. 32.4% Na
22.6% S
45.0% O
6. 29.1% Na
40.5% S
30.4% O
7. 77.7% Fe
22.3% O
8. 72.4% Fe
27.6% O
9. 70.0% Fe
.
30.0% O
Percent Composition and Molecular Formula Practice
1)
What’s the empirical formula of a molecule containing 65.5% carbon, 5.5%
hydrogen, and 29.0% oxygen?
2)
If the molar mass of the compound in problem 1 is 110 grams/mole, what’s the
molecular formula?
3)
What’s the empirical formula of a molecule containing 18.7% lithium, 16.3%
carbon, and 65.0% oxygen?
4)
If the molar mass of the compound in problem 3 is 73.8 grams/mole, what’s the
molecular formula?
Chemistry- Unit 7
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EXPLORE: (90 minutes)
Determination of the Empirical Formula for Zinc Chloride
In this activity, students will experimentally determine the empirical formula for zinc
chloride. A pre-lab section is provided to prepare students for calculations and
procedures of the lab.
Essential Question:
How can you experimentally determine the formula for a hydrate?
Determination of the Empirical Formula for Zinc Chloride
Name:__________________________________________ Course______ Period:____
Prelab
Show your Calculations for the following problems
1. How many grams of sulfur would combine with 26.1 g of potassium to form K2S?
2. A 1.31g sample of sulfur was reacted with chlorine to form 4.22g of compound. What
is the empirical formula of the compound?
3. 0.256g of a lanthanum oxide was decomposed and produced 0.219g of lanthanum.
What is the empirical formula of the compound?
4. What is the purpose of heating the empty evaporating dish and watch glass?
5. What is the purpose of the watch glass? Why is it not necessary to know the
individual masses of the evaporating dish and watch glass?
Determination of the Empirical Formula for Zinc Chloride
Chemistry- Unit 7
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24
Objective: To experimentally determine the empirical formula for the product of the
reaction between metallic zinc and hydrochloric acid by recovering the product by
evaporating it to constant dry mass.
Introduction: When zinc metal is reacted with hydrochloric acid, hydrogen gas is
released and an aqueous solution of zinc chloride is produced. The zinc chloride
solution when evaporated to dryness leaves a pure salt product. In order to calculate
the simplest whole number ratio of chlorine to zinc or empirical formula for zinc chloride,
we need to use the initial mass of zinc and the solid product formed. It is important that
all of the zinc used be reacted and that large excess amounts of hydrochloric acid not
be used. Then, from the mass of zinc consumed and the total mass of product formed,
the masses of both zinc and chlorine may be determined. Converting these to number
of moles and then calculating the simplest whole number ratio, by dividing by both by
the smaller of the two numbers, gives the empirical formula. If either of the numbers is
non-integral, then multiply by the smallest number required to give two whole numbers
as subscripts in the formula unit: ZnxCly.
Procedure
1. Mass an empty weighing boat to the nearest 0.01g. Place approximately two grams
of granular zinc metal in the weighing boat and mass them to the nearest 0.01g.
Calculate the mass of the zinc. Record the masses in the data table.
2. THERE SHOULD BE NO OPEN FLAMES IN THE CLASSROOM DURING THIS
REACTION!! Place the zinc in a large test tube and add 15.0mL of 6M hydrochloric
acid. Observe and record the indications of reaction initially. Remember that we do not
want an excess of zinc and we want most of the hydrochloric acid to be consumed. If
any zinc remains, add a small amount of HCl to react all of the zinc.
3. Mass a clean, dry evaporating dish and watch glass to the nearest 0.01g and record
their mass in the data table.
4. CAREFULLY pour the zinc chloride solution into the evaporating dish. Rinse the tube
twice with 1-2 mL of distilled water to transfer the rest of the zinc chloride solution to the
dish. A couple of small rinsings are better than one large rinsing.
5. Cover the dish with the watch glass and carefully heat the dish on a hot plate to
evaporate all the water. Try not to splatter the solution, which will result in a loss of the
compound. The first solid to appear will be the zinc chloride dihydrate. To remove the
water from the hydrate, heat the solid until it melts. Once it is completely molten,
remove from heat and let the dish cool to room temperature. Record the mass of the
dish and compound. Depending on how rapidly you did the initial heating, you may see
only the molten compound and never see the hydrate. Do not overheat the molten
compound or it will vaporize.
6. To ensure that the water has completely evaporated, reheat the dish and compound
Chemistry- Unit 7
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for a couple of minutes. Cool the dish to room temperature and remass. If the mass is
not within 0.1g of the previous mass, repeat the procedure until this is achieved.
7. Complete the calculations of the empirical formula.
Chemistry- Unit 7
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26
Determination of the Empirical Formula for Zinc Chloride
Name:__________________________________________________ Course:_____
Lab Partner:______________________________________________ Period:____
Data Table
Mass of weighing dish and zinc metal
Mass of empty weighing dish
Mass of zinc metal
Mass of the empty evaporating dish and watch glass
Mass of the evaporating dish, watch glass, and zinc chloride
after the first heating
Mass of the evaporating dish, watch glass, and zinc chloride
after the second heating
Mass of zinc chloride formed
Mass of chlorine in the compound
Moles of zinc in the compound
Moles of chlorine in the compound
Mole ratio of chlorine to zinc in the compound
Empirical formula for zinc chloride
g
g
g
g
g
g
g
g
mol
mol
Show your calculations for the above.
Answer the Following Questions
1. For each of the following, state the effect (increase, decrease, no change) on the
calculated mass of chlorine and the mole ratio of chlorine to zinc. Explain the effect in
terms of the mass of the zinc chloride and the mass of chlorine. Assume that your
original mass of zinc is correct.
a. All the reaction mixture was not transferred to the evaporating dish.
b. All the water was not evaporated during the heating process.
Chemistry- Unit 7
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27
c. Some of the reaction mixture splatters out of the evaporating dish during the heating
process.
d. Some of the zinc chloride is lost by vaporizing the solid due to overheating the solid.
e. Soot for the burner is deposited on the evaporating dish during heating to evaporate
the water.
f. Water is adsorbed by the zinc chloride during the cooling process but before the
compound is massed.
Conclusions:
What was your empirical formula? What possible sources of error might have
affected your results and how might they have affected it?
EXPLAIN:
Have students present their answers and calculations from the Determination of the
Empirical Formula for Zinc Chloride lab to the class. Instruct them to explain their
reasoning.
EXPLORE: (60 minutes)
Water in a Hydrate
In this activity, students will develop a procedure to determine the mass of water in a
known mass of hydrate. After the lab, the teacher should make sure students know
how to calculate the % of water in a hydrate (follow-up from % composition).
Essential Question:
How do you determine the percentage of water in a hydrate?
Chemistry- Unit 7
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28
Water in a Hydrate
Many compounds appear to be dry, but when they are heated, large amounts of water
are released. The water molecules are part of the crystalline structure and are weakly
bonded to the ions or molecules that make up the compound. Such compounds are
known as hydrates, meaning that they contain water. The solid that remains when the
water is removed, is referred to as the anhydrous salt, or anhydrate.
Hydrate + heat  anhydrate + water
Usually the amount of water present in a hydrate is in a whole number mole ratio
to the moles of anhydrate. An example is magnesium sulfate heptahydrate, MgSO 4 
7H2O. This formula indicates that seven moles of water are combined with one mole of
magnesium sulfate in the crystalline form.
Pre-lab
1. Define: (a) hydrate; (b) anhydrous salt or anhydrate.
2. What is the relationship between the mass of the hydrate and the mass of the
anhydrate + water?
3. The formula for calcium sulfate dihydrate is CaSO4  2H2O. What is the mole ratio of
water to the calcium sulfate?
4. Calculate the molar mass of CaSO4  2H2O. (Hint: CaSO4 + 2H2O)
5. Calculate the mass of the water in CaSO4  2H2O.
6. Calculate the percent mass of water in CaSO4  2H2O.
In this investigation you will determine the percent water in an unknown hydrate. To do
this you must:
 Develop a procedure to determine the mass of water driven off when a known
mass of the hydrate is heated. You will be using copper(II) sulfate. DO NOT
USE more than 1 – 2 g for your sample.
 Create a table to record the necessary data.
Requirements for your procedure:
 Heat the evaporating dish before placing any substance in it.
 Allow the evaporating dish to cool for a couple of minutes before measuring its
mass after heating.
 Measure the mass of the anhydrous salt immediately after cooling for about 2
minutes.
 Continue to heat your hydrate until you are SURE that ALL of the water has been
driven off; that is, until you reach a constant mass. Make sure you include
places for these measurements in your data table.
Data Analysis
1. Calculate the mass of the hydrate you started with.
Chemistry- Unit 7
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29
2.
3.
4.
5.
6.
Calculate the mass of water lost.
Calculate the percent of water in the hydrate.
Calculate the moles of water lost.
Calculate the mass of the anhydrous salt left.
Based on the molar mass of the anhydrous salt that is given, calculate the moles of
anhydrous salt.
7. Determine the smallest whole number ratio of moles of anhydrous salt to moles of
water by dividing each by whichever moles are the smaller.
8. Write the formula for the hydrate.
9. Compare your results with other members of your class. Are there differences?
What reasons could account for these differences.
Critical Thinking
1. Explain what effect the following errors would have on the value of the percent water
in the hydrate.
a. The hydrate was not heated long enough to drive off all of the water.
b. A damp evaporating dish was used, and it was not dried before adding the
hydrate.
2. Predict what would happen if you added a few drops of water to the anhydrous salt
remaining at the end of this experiment.
3. Dessicating agents are found in some optical equipment packages as well as some
food products. What are dessicating agents? What substances are used for this
purpose?
EXPLAIN:
Have students present their answers and calculations from the Water in a Hydrate lab to
the class. Instruct them to explain their reasoning.
EVALUATE:
Sample Assessment Questions:
Unit
7
Goal/
RBT
Tag
3.02
Questions
C3
A.
B.
C.
D.
1. Calculate the number of molecules in 36.0 g of H2O.
3.01 x 1023 molecules
6.02 x 1023 molecules
1.20 x 1024 molecules
3.90 x 1026 molecules
2. Determine the empirical formula of a compound which has
Chemistry- Unit 7
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30
C3
the following chemical analysis by mass: 48.64% carbon, 8.16%
hydrogen, and 43.20% oxygen.
A.
B.
C.
D.
C3H6O2
C2H3O
CH2O
CHO
EVALUATE: (45 minutes)
Below are sample test items obtained from the WIZARD test bank developed by
Eduware™ that can be used to allow students to assess their understanding and
abilities and allow the teacher to evaluate the students understanding of key concepts
and skill development for this unit.
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