Empirical and Molecular Formulas: Crime Scene Investigation
Purpose:
The purpose of this lesson is to introduce and supply opportunities to practice
empirical and molecular formula problems. Additional practice will be provided
using a popular issue – forensic issue. Students will be able to determine empirical
and molecular mass when given either percentage compositions or gram contributions
of each element in a compound in addition to molar mass.
Standards Addressed:
CH.1 The student will investigate and understand that experiments in which variables are
measured, analyzed, and evaluated produce observations and verifiable data. Key
concepts include use of appropriate technology including computers and graphing
calculators for gather data and communicating results; and construction and defense of a
scientific viewpoint (the nature of science).
CH.2 The student will investigate and understand that the placement of elements on the
periodic table is a function of their atomic structure. The periodic table is a tool used for
the investigations of average atomic mass, mass number, and atomic number and
chemical and physical properties.
CH.3 The student will investigate and understand how conservation of energy and
matter is expressed in chemical formulas and balanced equations. Key concepts
include writing chemical formulas such as molecular and empirical formulas.
NESF. As a result of activities in grades 9-12, all students should develop
understanding of personal and community health, natural resources, and
environmental quality.
Materials:
crutch
cup
empty pill bottle
tape for body outline on floor
caution tape if available
student notes
"Case Profiles"
Resources:
http://www.toxassociates.com/poisonous_compounds.htm
http://dbhs.wvusd.k12.ca.us/webdocs/Mole/EmpiricalFormula-Part2.html
Safety:
Always follow classroom safety rules. No additional concerns are present.
Time:
This lesson plan was designed for two 50-minute class periods.
Procedure:
I.
Powerpoint presentation on empirical and molecular
formulas. (See EmpiricalMolecularFormulaNotes.ppt)
A.
What is an empirical formula?
1.
Guided example
2.
Independent example
B.
What is a molecular formula?
1.
Easy guided example
2.
Difficult guided example
3.
Independent Example
II.
Crime Scene Investigation
A.
Students will be given a "Case Profile" that sets up the scene of
the crime and four possible causes of death. Students are to calculate the empirical
and molecular formulas for each scenario and then identify the chemical. Once this is
completed, they may access the autopsy report for confirmation.
B.
Students may examine the "crime scene" in the back of the
classroom for additional clues.
Crime Scene Investigation: Case Profile
The victim in the following case is a 35-year old white male named
Tony DeMoy. Initial investigators say they found several signs around the death site
that suggest foul play. Four possible causes of his untimely death have been
suggested by his wife who has been ruled out as a suspect because of a proven
alibi. Your task is to identify who and what killed Tony DeMoy.
1.
An unlabeled drug bottle was found in the medicine cabinet
of DeMoy's bathroom. A 532.99 g sample of the drug was analyzed and found to
contain 346.12 g C, 23.98 g H, 108.52 g O, and 54.36 g S. The molar mass of the
compound was found to be 314.38 g/mol. It may be a drug prescribed for
osteoarthritis. Coincidentally, DeMoy's arthritis has become so intolerable within
recent weeks that he's had to rely on the aid of a crutch for mobility. Prolonged
ingestion of this drug is thought to lead to heart attack. Demoy's doctor, Finley Finch
has a quiet reputation of prescribing dangerous drugs to "dispose of" unwanted
patients. Lately,DeMoy has been gaining more recognition in the community due to
his latest research on aromatic hydrocarbons. Could Finch's jealousy be the cause of
TonyDeMoy's death?
2.
Several of DeMoy's full or partial fingerprints were retrieved from a tipped
cup found near the crime scene. A faint almond scent was detected around the
rim. A 823.15 g liquid sample of the compound was found to contain 3.74% H,
44.43% C, and 51.83% N. The molar mass was determined to be ~27.03 g/mol. A
friend of DeMoy's noticed a disgruntled neighbor, Shay Lemarck, uncharacteristically
eager to provide refills. Could this be the answer?
3.
DeMoy's wife mentioned that the osteoarthritis had become so debilitating as
of late that he was "popping painkillers like candy". Certain painkillers are even more
dangerous in large quantities than others. One indication that this may be the cause of
death was a partially full acetaminophen bottle found on the scene. Chemical analysis
of one pill (1454.10 g) revealed 924.08 g C, 87.39 g H, 134.80 g N, and 307.83 g
O. The molar mass of the drug is known to be 151.18 g/mol. Could it be that the
crime scene is not a crime scene at all?
4.
DeMoy's latest and most brilliant research was on a specific type of aromatic
hydrocarbon. DeMoy, being a paranoid individual, wouldn't specify which one, but a
453.28 g sample was shown to contain only carbon and hydrogen. Hydrogen
contributed 7.76 % of the total mass and the molar mass was determined to be ~78.12
g/mol. The laboratory had a carefully regulated air flow to prevent high-level
inhalation. DeMoy periodically mentioned to his wife that he suspected his lab
partner, Kasey Hatterson, was altering the flow rates. Curiously, without DeMoy, the
published research could mean thousands of dollars for Hatterson. With money as
motive, could this solve the mystery?
When you have calculated the empirical and molecular formulas for each scenario and
identified each substance, you may access the autopsy report to confirm the cause of
death. Who, if anyone, killed Tony DeMoy?
Name __________________________ Period _________ Date _______
Empirical and Molecular Formula Notes
Empirical Formula
A formula that gives the simplest whole-number ratio of the atoms of each
element in a compound.
Chemical Formula (Molecular Formula)
H2O2
C6H12O6
CH3O
CH3OOCH = C2H4O2
Empirical Formula
HO
Determining Empirical Formulas
Steps:
1. Find mole amounts.
2. Divide each mole by the smallest mole.
Rhyme:
Percent to mass
Mass to mole
Divide by small
Multiply 'til whole
Determine the empirical formula for a compound containing 2.128 g Cl and 1.203 g
Ca.
A compound weighing 298.12 g consists of 72.2% magnesium and 27.8% nitrogen by
mass. What is the empirical formula?
Molecular Formula
The formula that gives the actual number of atoms of each element in a
compound.
Determining Molecular Formulas
Steps:
1. Find the empirical formula.
2. Calculate the Empirical Formula Mass.
3. Divide the molar mass by the “EFM”.
4. Multiply empirical formula by the factor.
Find the molecular formula for a compound whose molar mass is ~124.06 and
empirical formula is CH2O3.
Find the molecular formula for a compound that contains 4.90 g N and 11.2 g O. The
molar mass of the compound is 92.0 g/mol.
Putting EVERYTHING together
A 528.39 g compound containing only carbon, hydrogen, and oxygen is found to be
48.38% carbon and 8.12% hydrogen by mass. The molar mass of this compound is
known to be ~222.25 g/mol. What is its molecular formula?
Autopsy Report
Tony DeMoy
Autopsy Report or Clues at Crime Scene
1.) It is not the substance whose sum of the subscripts on the molecule or formula is 20.
2.) It is not the substance whose sum of the subscripts on the molecular formula divided by
the subscript on oxygen is 9.
3.) It is not the substance whose empirical and molecular formulas are different.
He had high levels of Rofecoxib in his blood. Explain what killed Tony DeMoy.