Instructors Guide: Percent Composition
Standards Connections
Connections to NSTA Standards for Science Teacher Preparation
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C.3.a.6 – Mole concept… and laws of composition.
C.3.c.31 – Mathematical… and skills….
C.1.3 – Measurement as a way of knowing and organizing observations of
constancy and change
Connections to the National Science Education Standards:
Structure of Atoms
• Matter is made of minute particles called atoms… These components have
measurable properties, such as mass….
• … A compound is formed when two or more kinds of atoms bind together
chemically.
Connections to New York Chemistry Core Curriculum:
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The formula mass of a substance is the sum of the atomic masses of its atoms.
The molar mass (gram formulas mass) equals one mole of that substance. (3.3e)
The percent composition by mass of each element in a compound can be
calculated mathematically. (3.3f)
… A chemical compound can be represented by a specific chemical
formula….(3.1cc)
Hints for Facilitation
Model #1: The answers in the model are arbitrary; make the numbers personal to your own
class.
Model #2: The lab was actually performed and data was collected. The gum used was
Eclipse Spearmint. Other types of gum will probably net different results. If using sugarfree gum the results will show very little loss of mass, and may even show an increase in
mass. If you choose to do the experiment with your students, remember to take proper
precautions when using food in a chemistry classroom. Be very careful to keep the gum on a
clean surface when using chemical balances. Use paper towels to ensure no contamination.
Key Question #2 and 3:
A Limiting response to a student question would be – Saliva would make the mass heavier,
right? What would happen to the percentage then?
A Promoting response to a student question would be – Think about what would happen to
the mass if the saliva wasn’t there?
Exercises #3 – 6:
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
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Percent Composition
Teachers may want to remind students what the subscripts and parenthesis mean when
discussing formula masses for compounds
Suggestions for appropriate demonstrations, manipulatives, etc., that will
enhance the activity.
Demonstration: (Practice this before doing for the class!)
This activity deals with the percent composition by mass of the water that exists in a bag of
microwave popcorn. Popcorn is a good analogy for loss of water in a hydrate, thus can be
expanded to be used as such. This demonstration can be done in a lab room or lecture room
with access to a microwave. Also needed is a bag of microwaveable popcorn.
Pre – demonstration questions:
1. Ask students if they know what makes popcorn ‘pop’? (water)
2. What role does the microwave play? (Heats up the water in the kernel which
ultimately expands to stress out the integrity of the pop corn shell)
3. Is it possible to figure out how much water is contained in the bag of
popcorn? (yes)
4. You can ask students to devise a procedure in order to figure out how
much water is in the bag of popcorn. (Subtract the bag before popping and
after popping (open bag) – the water is missing)
5. Should the bag be heavier or lighter? (lighter)
6. (Eat! – But if in a lab room take appropriate precautions.)
Calculations:
• Have students find the percentage water in the popcorn.
Expansion:
• This can be expanded to compare popcorn brands – economic twist.
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
2/8
Percent Composition
Percent Composition
Why?
Congratulations! On your last chemistry exam you answered 38 of 40
questions correctly. You achieved a grade of 95%! The percentage grade on your
exam indicates the part of the exam you answered correctly. The part of the exam
that you answered incorrectly is only 5%, or 2 questions. Percentage is also a useful
tool in chemistry. The percent composition by mass of a compound represents the
percent that each element in a compound contributes to the total mass of the
compound. A chemist often compares the percent composition of an unknown
compound with the percent composition calculated from the formula of a known
compound. If the percentages agree it may confirm the identity of the unknown.
Learning Objectives
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Understand what is meant by percent composition by mass of a compound.
Ability to determine the percent composition by mass of a compound.
Success Criteria
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Calculate the percent composition by mass of the elements in a variety of
chemical compounds.
Explain the procedure used to determine the percent composition of a compound.
Use percent composition to arrive at conclusions regarding compounds.
Prerequisites and Vocabulary
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Symbols and names of elements
Atomic mass
Molar mass
Chemical formulas
Hydrated crystal
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
3/8
Percent Composition
Task 1
Using the students present in class today, complete the data table in Model 1.
[Note that Percent = (part/total) x 100.]
Model 1
Item
Students
Total # of
Students
25
Total # of
Girls
17
Total # of
Boys
8
Percent
Girls
68%
Percent
Boys
32%
The calculation below is based on the theoretical data
shown in the chart.
Key Questions
1. Describe how one could calculate the percent of boys present today using the
data in the chart.
Number of boys / total number of students) x 100
2. What is the percent of boys present today, based on the data in the chart?
8
× 100 = 32%
25
Task 2
When you chew a piece of gum, mass is lost as the sugar dissolves. After the
sweetness is gone, only the ‘gum’ remains. Given a piece of gum to chew, each
student will collect data to complete the data table in Model 2.
As a team, propose and describe below an experimental procedure that will
permit you to collect the data needed to determine the percent of sugar in the gum.
Do not proceed with the experiment until your instructor has approved your
procedure.
Answers may include:
Materials:
• Balance
• Gum
• Weigh paper or napkin
Procedure
• Mass of gum before (in g)
• Chew gum for a period of time (ex. 10 minutes)
• Mass of gum after (in g)
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
4/8
Percent Composition
Use the space below to collect your data – put into Model 2 on the next page.
Data:
Mass of gum before:
1.50 g
Mass of gum after 10 minutes:
0.70 g
Difference (mass before – mass after):
0.80 g
Model 2
Item
Total Mass
Mass of
sugar
Mass of
gum
Percent
sugar
Percent
gum
Chewing
Gum
1.50 g
0.80 g
0.70 g
53%
47%
Key Questions
1. Show the set-up used to determine the percent by mass of sugar in the chewing gum.
0.80 g
× 100
1.50 g
2. What is the percent of sugar in the chewing gum?
53%
3. At the end of this experiment the gum is wet with saliva. How does the presence
of the saliva influence the percentage reported in Question 2?
Saliva would increase the reported percentage of sugar
4. What method could be used to eliminate the error introduced by the presence of
the saliva?
Dry the gum before determining the final mass.
Model 3
Item
Total Mass
CH4,
methane
16.0 g
Mass of
carbon
12.0 g
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
Mass of
hydrogen
4.0 g
Percent
carbon
75.0%
Percent
hydrogen
25.0%
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Percent Composition
Key Question
1. What is the mass of carbon in 16.0 g of methane?
12.0 g C
2. Show the set-up that would be used to determine the percent by mass of carbon
in methane in order to arrive at the answer shown in the Model 3 table.
12.0 g
16.0 g
× 100
3. What information do you need in order to determine the percent composition by
mass of sodium in sodium chloride?
Formula for sodium chloride
Atomic mass of sodium and chlorine
OR Mass of sodium or chlorine in a known mass sample of
sodium chloride
Exercises
Show set-up used to solve each exercise. Round atomic masses to the
nearest tenth.
1. Write the general mathematical formula that can be used to calculate the
percent composition by mass of any substance?
mass ~ of ~ part
× 100
mass ~ of ~ whole
2. Use the atomic masses of the elements to determine the percent composition by mass of
sodium and chlorine in sodium chloride.
Sodium = Na = 23.0 g
Chlorine = Cl = 35.5 g
Sodium Chloride = NaCl = 58.5 g
Sodium:
23.0 g
× 100 = 39.3%
58.5 g
Chlorine:
35.5 g
× 100 = 60.7%
58.5 g
3. Use the atomic masses of the elements to find the percent composition of
hydrogen in H2O.
Hydrogen = H = 1.0 g x 2 H = 2.0 g ; Oxygen = O = 16.0 g
Water = H2O = 18.0 g
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
6/8
Percent Composition
Hydrogen:
2.0 g
× 100 = 11.1%
18.0 g
4. Determine the percent composition of oxygen in potassium chlorate, KClO3
• Potassium = K = 39.1 g
• Chlorine = Cl = 35.5 g
• Oxygen = O = 16.0 g x 3 O = 48.0 g
• Potassium Chlorate = KClO3 = 122.6 g
Oxygen:
48.0 g
× 100 = 39.2%
122.6 g
5. Determine the percent composition of phosphorus in calcium phosphate,
Ca3(PO4)2.
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Calcium = Ca = 40.1 g x 3 = 120.3 g
Phosphorus = P = 31.0 g x 2 = 62.0 g
Oxygen = O = 16.0 g x 8 = 128.0 g
Calcium Phosphate = Ca3(PO4)2 = 310.3 g
Phosphorus:
62.0 g
× 100 = 20.0%
310.3 g
6. Copper (II) sulfate pentahydrate, with the formula CuSO4· 5H2O, has a deep
blue color. When heated to remove the water, the crystals crumble and turn
white. What is the percentage, by mass, of water in copper (II) sulfate
pentahydrate crystals?
% Water:
90.0 g
× 100 = 36.0%
249.5 g
Problems
Show set-up used to solve each problem.
1. The percent of oxygen in a colorless liquid is determined to be 94.1%. Is this
liquid water or hydrogen peroxide (H2O2)?
16.0 g
Oxygen in water (H2O):
× 100 = 88.8%
18.0 g
Oxygen in hydrogen peroxide (H2O2):
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
32.0 g
× 100 = 94.1%
34.0 g
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Percent Composition
2. What is the percent composition of a compound that contains 8.1 grams of nickel
and 2.2 grams of oxygen in a 10.3-gram sample?
Nickel:
8.1g
× 100 = 78.6%
10.3 g
Oxygen:
2.2 g
× 100 = 21.4%
10.3g
3. Nicotine, the addictive drug in cigarettes, contains 74.0% carbon, 8.6% hydrogen,
and 17.3% nitrogen. What mass of each element can be recovered from a 55.0gram sample of nicotine?
Carbon: 74.0% x 55.0 g = 40.7 g
Hydrogen: 8.6% X 55.0g = 4.7 g
Nitrogen: 17.3% X 55.0 g = 9.57 g
©POGIL 2005, 2006
Revised by E. Graham, L. Giloni, B. Black and B. Horan
Edited by Linda Padwa and David Hanson, Stony Brook University
8/8