Thornburg 2011
Background
A crime has been committed and you, a forensic chemist, have been asked to help solve the crime. Fred
Flintstone, a popular student at Bedrock high School, was found dead on the floor of Barney Rubble’s donut
shop. Inspector Smuts, the lead detective assigned to the case, has gathered some critical evidence for you to
review. A clear liquid was found next to the body. When this liquid was accidentally mixed with salt on the
floor during the initial investigation, it produced an opaque substance. Flintstone was known to have had a
violent reaction to potassium iodide, and there was widespread use of silver nitrate in the laboratory by
Flintstone’s colleagues. Silver nitrate can be highly toxic if ingested. Inspector Smuts has discovered
additional evidence, which is presented in the following summary.
Professor Baker, an eccentric chemistry teacher at Bedrock, was working in his laboratory,
with silver nitrate solutions of molarities 0.20 to 0.30. Flintstone was Professor Baker’s
worst student, arriving late to class, continually breaking glassware, and generally making
a big mess of the professor’s laboratory. Inspector Smuts, having never taken a
chemistry course, did not consider this to be a motive for murder.
Larry the Prairie Dog, who suffers from asthma, has a prescription for potassium
iodide in his locker. His pharmacist has assured Inspector Smuts that Larry’s potassium iodide solution has a
molarity in the range of 0.05M to 0.15M. Due to a dispute with Flintstone over a stolen I-pod last week, the
police have considered Larry as a possible suspect.
Gary the Prairie Dog, Larry’s twin brother, is an amateur photographer. Gary had solutions of silver nitrate
between 0.05M and 0.15M in the darkroom. Pictures of Flintstone were discovered in Gary’s gym locker. It is
widely known across campus that Gary was angry with Flintstone. Gary believed Flintstone was responsible
for his being cut from the basketball team. This leads authorities to consider Gary as a suspect.
Spanky the Bear, who is still angry with Flintstone for taking his girlfriend Sheena, was found in the biology
lab. He was administering a solution of potassium iodide to his pet guinea pig. His veterinarian has informed
the police that the strength of the solution was greater than 0.20M. Inspector Smuts believes that Spanky was
well aware of Flintstone’s violent reaction to potassium iodide.
Rosie the bear, Spanky’s sister, had silver nitrate solutions in excess of 0.30 M, in her desk in the physics
laboratory. Rosie, who was well known to have spent many hours of her spare time in the lab developing
explosives, also despised Flintstone’s liberal politics. Rosie served as the ASB president of the school until
Flintstone led a revolt and ousted her from office.
Bucky the Badger, Flintstone’s neighbor, had potassium iodide in his garage with a concentration between
0.15M and 0.20M. Bucky claims he used the potassium iodide in the repair of his vintage corvette, which
Flintstone damaged while parking his own car. Although they have been neighbors for several years, Bucky
and Flintstone were reportedly seen arguing over Bucky’s car.
Your Task
You can obtain a sample of the murder weapon found at Flintstone’s side by asking your teacher for it. If you
can determine whether it is silver nitrate or potassium iodide, then you can narrow the list of suspects
considerably. And if you can determine the molarity of the unknown solution, then you can solve the case.
Who was the murderer? What was the murder weapon? What was its molarity? You can determine the
answers to these questions by carefully and correctly completing the lab investigation that follows. Remember,
on the basis of your evidence, a murderer might be set free or an innocent person executed
Thornburg 2011
Suggestions
1. Use your knowledge of double replacement reactions and solubility rules to determine how each of the
possible murderous solutions would react with the known solutions provided. Perform the reactions and
assess the results.
2. Once you have figured out which of the possible solutions the unknown is, use your knowledge of
separation of mixtures, and the definition of molarity to determine the approximate molarity of the
solution.
3. After you have determined the identity of the murderous solution and its molarity you should be able to
conclude which of the suspects is the murderer and where the murder was committed in order to write
up your report for the court trial.
Materials
Test solution #1 – 0.1M sodium chloride
Test solution #2 – 0.1M sodium carbonate
Well plate
Sample of each murder weapon
Magnifying glass
Medicine droppers
Procedure – Day 1
1. Determine the 4 possible reactions between the possible murder weapons (potassium iodide and silver
nitrate) and the test solutions (sodium chloride and sodium carbonate).
Table #1: Possible Reactions
Write the complete balanced equation for the 4 possible double replacement reactions in the table
below
1
2
3
4
2. Perform each of these reactions by combining 3 drops of each of the reactants in one of the wells on
your well plate.
3. Record your results and observations in your data table #2.
4. Clean Up: Dispose of any opaque solutions in the waste beakers in the classroom. Otherwise,
thoroughly wash and dry all glass ware. Be sure to clean off your lab table and to neatly organize your
lab equipment.
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Table #2: Observations of 4 Double replacement Reactions
Reaction #
Possible murder weapon
Products formed
reacted
Analysis Questions:
1. Based on your evidence above, which chemical is the murder weapon? Explain.
2. Which suspects can be eliminated based on this evidence?
3. Who still remains as a suspect?
Observations & Description of
Products
Thornburg 2011
Background
As you recall, you have been assigned the task of determining who murdered Fred Flintstone. In part 1 of your
investigation, you determined which chemical was used to murder Flintstone. Today you will use your
knowledge of stoichiometry and molarity to determine the molarity of the murder weapon and ultimately identify
the murderer.
Before you test the murder weapon to determine its molarity, you will do some practice experimentation in part
A of the lab, in order to refine your technique. In part B, you will use the same principles form part A to plan
out your experimental procedure for determining the molarity of the murder weapon. Good Luck – And
remember… the fate of justice lies in your hands!!!
Materials
Evaporating dish
Graduated cylinder
0.5 M HCl
O.5 M NaOH
Hot Plate
Medicine droppers
Balance
Procedure
1. Weigh your evaporating dish and record its mass here: __________ g
2. Using a graduated cylinder, measure out 15 mL of 0.5 M HCl. Place the HCl in your evaporating
dish.
3. Using your graduated cylinder, measure out 10 mL of 0.5 M NaOH and place it into the dish.
4. Heat the dish and its contents on the hot plate (at a setting of 7) until all liquid is evaporated.
5. Record the mass of the dish and the powder that is left over: _________ g
6. Subtract the mass of the dish from the mass of the dish + powder to determine the mass of the
powder. Mass of the powder: __________ g
7. Clean up all of the lab equipment and lab area. Wash glassware thoroughly in the sink.
Data & Conclusions
1.
Write the balanced equation for this reaction in the space below:
2. Using the given molarity of the solution, calculate the number of moles of HCl you used.
_________ mol
3. Using the given molarity of the solution, calculate the number of moles of NaOH you used.
_________ mol
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4. Knowing how many moles of each were used, which reactant is the limiting one?
5. Use stoichiometry and your answers to the questions above to calculate the theoretical number of
moles of salt that could be produced in your reaction. (Remember – the limiting reactant was used
up first, so you must use the number of moles of the limiting reactant for your calculations – The
reactant that will produce the least mass of product)
___________ mol
6. Based on your stoichiometry, how many grams of salt should have been produced?
_________ g
7.
How many grams of salt were actually produced in your experiment?
_________ g
8. What was your percent error?
_________ %
9. Explain, inn detail, any sources of error to account for your answer to #8.
Thornburg 2011
Background
Now that you have had some practice, you will get back to the business of solving the murder of Fred
Flintstone. You have already determined the murder weapon from your lab work last week. Now you will get
down to the business of determining the molarity of the murder weapon, and ultimately, who murdered
Flintstone.
Purpose
To determine the molarity of the solution used to kill Fred Flintstone.
Materials
Evaporating dish
Graduated cylinder
A 8.0 mL sample of the murder weapon
Test solution #1 – 0.1M sodium chloride
Hot Plate
Medicine droppers
Balance
On the following page, you will write your own procedure for how your group will determine the molarity of the
solution used to murder Flintstone.
Suggestions:
 Write the balanced equation for the reaction you will perform to determine the molarity of the murder
weapon:
 Your basic plan should be as follows:
o React the murder weapon with the test solution
o Boil the products to evaporate 1 of the 2 products
o Measure the mass of the solid product that is produced
o Do some stoichiometry using the mass of the solid product to figure out the moles of the murder
weapon reacted
o Use the moles of murder weapon reacted to determine the molarity of the murder weapon and
identify the killer!!!
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Procedure for B
1.
____________________________________________________________________________
2. ____________________________________________________________________________
3. ____________________________________________________________________________
4. ____________________________________________________________________________
5. ____________________________________________________________________________
6. ____________________________________________________________________________
7. ____________________________________________________________________________
8. ____________________________________________________________________________
9. ____________________________________________________________________________
10. ____________________________________________________________________________
Data Table:
Calculations for Part B
Molarity of the Murder Weapon :
Based on your evidence, who murdered Flintstone?
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Names: _____________________________________
_____________________________________
_____________________________________
__________________________________
__________________________________
Period: __________
1. Based on Day 1 of the lab, what chemical did you determine to be the murder
weapon? ___________________________________
2. How did you know this was the murder weapon?
____________________________________
____________________________________________________________________________
____________________________________________________________________________
3. For today’s portion of the lab, write the balanced chemical equation for the chemical reaction you
performed:
________________________________________________________________________
4. Which product was a solid? _____________________________________________________
5. Show how you calculated the number of moles of the murder weapon :
____________ moles
6. Show how you calculated the molarity of the murder weapon:
Molarity: ____________
7. Based on your experimental evidence, who is the guilty suspect in the murder of Fred Flintstone?
_____________________________________________________________