Download Percent Composition And Chemical Formulas

Percent Composition And Chemical Formulas
by CHED on June 16, 2017
lesson duration of 0 minutes
under General Chemistry 1
generated on June 16, 2017 at 04:00 pm
Tags: Composition, Formulas, Percent
CHED.GOV.PH
K-12 Teacher's Resource Community
Generated: Jun 17,2017 12:00 AM
Percent Composition And Chemical Formulas
( 1 hour )
Written By: CHED on July 15, 2016
Subjects: General Chemistry 1
Tags: Composition, Formulas, Percent
Resources
Chemistry: An atoms first approach.
Zumdahl, S.S. &Zumdahl, S.A. (2012). Chemistry: An atoms first approach. Belmont, CA: Brooks/Cole Cengage
Learning.
Chemistry: The molecular science (4th ed.)
Moore, J.W., Stanitski, C.L. & Jurs, P.C. (2012). Chemistry: The molecular science (4th ed.). Belmont, CA: Brooks
Cole/Cengage Learning.
Chemistry. (12th ed.).
Chang, R. & Goldsby, K. (2016). Chemistry. (12th ed.). New York: McGraw-Hill.
Chemistry: Atoms first.
Burdge, J.& Overby, J. (2012). Chemistry: Atoms first. New York: McGraw-Hill.
Content Standard
The learners demonstrate an understanding of:
2. the relationship of percent composition and chemical formula
Performance Standard
The learners design using multimedia, demonstrations, or models, a representation or simulation of any of the
following:
1. atomic structure
2. gas behavior
3. mass relationships in;
4. reactions
1/5
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Learning Competencies
The learners calculate molecular formula given molar mass
The learners calculate the empirical formula from the percent composition of a compound
The learners calculate the percent composition of a compound from its formula
Introduction 5 mins
1. Introduce the learning objectives using any of the suggested protocol (Verbatim, Own Words, or Read-aloud)
1. Interpret the information provided by the chemical formula
2. Explain the procedure used to determine the percent composition of a compound
3. Explain the procedure used to determine the empirical formula of a compound given the percent
composition
4. Utilize molar mass data to obtain the molecular formula from the empirical formula
2. Present the keyword for the concepts to be learned:
1. Percent composition mass
3. Review empirical formulas and molecular formulas and give examples.
Percentage of the class planning (as first choice) 5 mins
1. Get total number of learners in class.
2. Get the number of learners who would like to pursue engineering in college. Get the percentage.
3. Get the number of learners who would like to pursue the sciences. Get the percentage.
4. Get the number of learners who would like to pursue mathematics. Get the percentage.
Instruction/Delivery 35 mins
I. Information from the chemical formula
What information can be obtained from a chemical formula? For example, what information can you get from the
formula of carbon dioxide, CO2?
1. The compound is made up of two elements, namely carbon and oxygen.
2. One molecule of CO2 is made up of one atom of carbon and two atoms of oxygen.
3. One mole of CO2 molecules will have one mole of C atoms and two moles of O atoms.
4. The ratio of the moles of C to the moles of O in CO2 is 1:2.
5. CO2 is composed of 27.29% carbon and 72.71% oxygen. The chemical formula provides the percent
composition of CO2.
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II. Percent Composition by Mass
The percent composition by mass is the percent by mass of each element in a compound. Mathematically,
For CO2,
The answer indicates that CO2 is composed of 27.29% C atom and 72.71% O atom.
Ask them to answer the following practice exercises:
1. Calculate the percent composition of NaCl.
2. The chemical formula of glucose is C6H12O6. Determine its percent composition.
3. Which element comprising Mg(OH)2 has the highest percentage by mass?
III. Empirical Formula from
from Percent Composition
The empirical formula of a compound can be calculated from the percent composition. Because percentage is given,
it is convenient to assume 100.00 grams of the compound.
Illustrate using the following examples:
1. A compound is found to consist of 7.81% C and 92.19% Cl. What is the empirical formula of the compound?
Assume 100.00 grams of the compound. The sample will therefore contain 7.81 g C and 92.19 g Cl. The grams are
converted to moles to get the ratios of the moles of the elements in the compound:
The compound is C0.650Cl2.601. But chemical formulas are expressed in whole numbers. Empirical formulas are
expressed as the lowest whole number ratio between the atoms. To convert to whole numbers, divide the number of
moles by the smallest value (that is 0.650).
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The empirical formula is C1Cl4 or CCl4.
2. A compound is found to consist of 43.64% P and 56.36% O. The molar mass for the compound is 283.88 g/mol.
What is the empirical formula and molecular formula of the compound?
Assume 100.00 grams of the compound. What is the mass of each element in 100.00 grams of compound?
P 43.64 g O 56.36 g
What are the moles of each element in 100.00 grams of compound?
Divide the mole values by the smallest value to get
The compound is PO2.5. But the subscripts are still not whole numbers. Multiply the subscripts by a factor to get the
smallest whole number. When multiplied by 2, the empirical formula is P2O5.
What is the molecular formula?
Compare the mass of the empirical formula to the molar mass:
Mass of P2O5 = 141.94 g/mol
Molar mass = 283.88 g/mol
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O ) or P4O10.
Therefore, the molecular formula is (P2 5 2
?Determine the % sodium in snack food 0 mins
Look at the food labels of some snack food like potato chips, peanuts, popcorn, etc. Fill up the table below.
_______________________1.
______________________ 2.
Identify your chosen snack food and brand.
Get the amount in grams of one serving of the snack food.
______________________ 3.
Get the amount of sodium in mg in one serving of the
snack food.
Obtain the % sodium by mass in one serving of snack
food.
______________________ 4.
Evaluation 15 mins
Answer the following questions. Place the answers in the space provided. Show calculations where applicable.
Observe the use of significant figures for calculations and indicate the appropriate units. Learners can use the periodic
table to answer the questions.
Aspirin has the molecular formula C9H8O4.
Answer key
___________________________ 1.What is the % C in
aspirin by mass in aspirin?
60.00%
___________________________ 2.What is the % O in
35.53%
aspirin by mass in aspirin?
___________________________ 3. An oxide of chromium Cr2O7
is made up of 5.20 g chromium and 5.60 g oxygen What is
the empirical formula of the oxide?
An oxide of nitrogen contains 63.1% oxygen and has a
N2O3
molar mass of 76.0 g/mol.
___________________________ 4. What is the empirical
formula for this compound?
___________________________ 5. What is the molecular N2O3
formula of the compound?
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