In this lab, students use molarity concepts to review limiting reactant concepts mathematically, conceptually, and graphically. They can then carry out a follow up investigation to identify an unknown using concepts learned in the first investigation.
AP Chemistry Curriculum Framework
This lab supports the following units, topics and learning objectives:
- Unit 3: Intermolecular Forces and Properties
- Topic 3.7: Solutions and Mixtures
- SPQ-3.A: Calculate the number of solute particles, volume, or molarity of solutions.
- Topic 3.10: Solubility
- SPQ-3.C: Explain the relationship between the solubility of ionic and molecular compounds in aqueous and nonaqueous solvents, and the intermolecular interactions between particles.
- Topic 3.7: Solutions and Mixtures
- Unit 4: Chemical Reactions
- Topic 4.2: Net Ionic Equations
- TRA-1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
- Topic 4.5: Stoichiometry
- SPQ-4.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.
- Topic 4.7: Types of Chemical Reactions
- TRA-2.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
- Topic 4.2: Net Ionic Equations
By the end of this lesson, students should be able to
- predict which chemical is the limiting reactant and verify their calculations with their findings.
- conduct a double replacement reaction that results in a precipitate.
- identify an unknown solution using similar skills to the an initial investigation.
This lesson supports students’ understanding of
- Limiting reactant
- Net ionic equation
- Percent yield
Teacher Preparation: 50 minutes
Lesson: 2 class periods for White Lab, 1 class period for Return of the White Lab
- Test tubes (8)
- Test tube rack
- 10-mL graduated cylinder
- Filter paper (5)
- Erlenmeyer flask
- 0.200-M potassium iodate solution
- 0.200-M barium chloride solution
Return of the White Lab
- Unknown solution (these are the parameters of each suspect)
- Professor Plum: 0.20-M to 0.30-M BaCl2
- Mrs. White: 0.050-M to 0.15-M KIO3
- Mr. Green: 0.050-M to 0.15-M BaCl2
- Mrs. Peacock: 0.20-M to 0.30-M KIO3
- Colonel Mustard: >0.30-M BaCl2
- Students must determine identity and concentration of unknown solution, so be prepared to provide necessary materials based on their experimental design
- Always wear safety goggles when working with chemicals in a lab setting.
- Barium iodate is a serious eye irritant and a mild skin irritant so take suitable precautions.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- I do the White Lab so I can do the Return of the White lab—a problem solving activity in which students design their own procedure and carry it out to solve the crime.
- See evaluation sheet for suggested grading rubric for the White Lab.
For the Student
You will react potassium iodate with barium chloride in a series of double replacement reactions. Barium iodate is insoluble and will form a precipitate. In each reaction, you will determine the mass of barium iodate formed, identify the limiting reactant, and calculate the % yield.
- Write the balanced equation for the reaction you will investigate.
- After reading the lab, complete this analysis. Attach all calculations.
|Mass of KIO3 in the test tube|
|Mass of BaCl2 in the test tube|
|Theoretical mass of KIO3 reacted|
|Theoretical mass of BaCl2 reacted|
|Name or formula of limiting reactant|
|Name or formula of excess reactant|
|Theoretical mass of KIO3 unreacted|
|Theoretical mass of BaCl2 unreacted|
|Theoretical yield of barium iodate|
|Theoretical molarity of KCl|
RUNNING THE REACTIONS
- Label five test tubes 1, 2, 3, 4, and 5 and place them in a test tube rack. Place your initials on a label on one of the test tubes.
- Prepare these combinations in the five numbered test tubes:
mL of 0.200-M KIO3
|2.00 mL||4.00 mL||6.00 mL||8.00 mL||10.00 mL|
mL of 0.2000-M BaCl2
PART B: USING FILTRATION TO ISOLATE THE PRECIPITATE
1. Number five pieces of filter paper 1–5 to correspond with the test tube numbers. Weigh each filter paper to two decimal places. Record the masses.
2. Place a clean rubber stopper in one test tube and shake vigorously.
3. Place a filter paper in the funnel and filter the solid from the solution. The potassium chloride is soluble and will filter through the paper and drip with the filtrate into the lower flask. Add the collected filtrate to the same test tube from which it was poured, swirl, and repour through the filter paper. Repeat until most of the precipitate is on the filter paper. Keep the filtrate until you are finished with Part C, step one.
4. After filtration, you have barium iodate on the filter paper and potassium chloride, dissolved in the liquid in the test tube. You may also have one of the reactants in excess in the test tube. If you leave the precipitate wet, when the water evaporates overnight, these impurities will be mixed with your precipitate. However, you can wash away most of these impurities with water. But you must be careful! barium iodate, which you do not wish to wash away, is slightly soluble in water (3.17 x 10-8 g/100 mL), so addition of water risks washing some of the desired product away. Thus you should add a small amount of water (5 mL), which will eliminate most of the impurities while leaving almost all of the barium iodate.
5. Wash the precipitate with 5 mL of water, twice, collecting the washings in an empty container, then remove the filter paper from the funnel and leave it in the test tube rack overnight to dry. The washings can go down the drain.
6. Repeat steps 2-5 for each test tube, cleaning the stopper between each test tube.
PART C AFTER FILTRATION
- After filtration, perform the following tests on the clear filtrate from the five test tubes and record your observations.
a) Pour some filtrate from test tube 1 and some filtrate from test tube 2 into a clean test tube. Record the color.
b) Pour some filtrate from test tube 4 and some filtrate from test tube 5 into a clean test tube. Record the color.
c) Now, combine the liquids from 1a and 1b in a third clean test tube. Record the color.
- Weigh the dry filter paper with the barium iodide precipitates. Record your masses.
- Discard the filter paper with precipitate in the trash. Wash the test tubes.
|a) Mixing filtrate 1 with filtrate 2|
|b) Mixing filtrate 4 with filtrate 5|
|c) Mixing combination a) with combination b)|
|Test tube number||1||2||3||4||5|
|Mass of barium iodate formed|
- On graph paper, plot the mass of potassium iodate used in each test tube (the reactant varied – x-axis) against the actual mass of barium iodate that formed (the product measured – y-axis).
a) Why does the graph rise? Why does it level off?
b) How does the graph indicate the excess reactant?
c) In which test tube, if any, were the reactants in the stoichiometric ratio predicted by the balanced equation? (If you did not conclude test tube 3, see your teacher for help.)
- Do the experimental results from parts C1 (a, b and c) agree with your prelab calculations and your graph about which reactant should have been in excess in each test tube? Explain how you know.
- Error analysis
|Test tube number||1
|Theoretical mass of barium iodate formed (from prelab)|
|Actual mass of barium iodate formed (from data table)|
Return of the White Lab
Miss Scarlet was found dead on the floor of the BALLROOM. Beside her body, police found a clear liquid, undoubtedly the murder weapon. Detectives were quick to ascertain that the murder weapon was one of two chemicals, barium chloride or potassium iodate. First it was known that Miss Scarlet had a violent allergic reaction to potassium iodate. Second, there was widespread use of a moderately toxic poison, barium chloride, by several members of the household. Police further discovered the following evidence.
Professor Plum, an eccentric chemistry teacher, had been working in the STUDY with barium chloride solutions of molarities in the range of 0.20 M to 0.30 M. It was naturally assumed that he was preparing for the White Lab, but now… Miss Scarlet was old Plum’s worst student, arriving to class late, being responsible for lots of broken glassware, and rarely cleaning up after her experiments. The policeman in charge, never having taught a chemistry class, did not seriously consider this to be a motive for murder.
Mrs. White, an asthmatic, has a prescription of potassium iodate in the BATHROOM. Her pharmacist assures you that her prescription is between 0.050 M and 0.15 M. That Mrs. White was presently rooming with Miss Scarlet’s exboyfriend led the police to suspect her. Neither she nor her boyfriend could be found in the LOUNGE for comment.
Mr. Green, a photographer, had solutions of barium chloride between 0.050 M and 0.15 M in the DARKROOM. Mr. Green apparently had been caught scarlet-handed at his own blackmail game. Pictures of him with Miss Scarlet were found hidden in Scarlet’s BEDROOM. Mrs. Green would be spared seeing the pictures if at all possible.
Mrs. Peacock, wearing the same dress as Miss Scarlett, was found in the STABLE, administering potassium iodate solution to her horse. When questioned, she admitted that her horse had a severe case of bronchitis. Her veterinarian informed detectives that the strength of the solution was between 0.20 M and 0.30 M. It is likely that she knew of Miss Scarlet’s violent allergy to potassium iodate because they were lab partners in Professor Plum’s chemistry class. Professor Plum recalled how angry Mrs. Peacock would get when Miss Scarlet came late to class, unprepared for the experiments, and how she invariably was sick the day lab reports were due.
Finally, there is Colonel Mustard in the CONSERVATORY. With barium chloride solutions in excess of 0.30 M, the Colonel makes fireworks. How he despised the left-wing politics of Miss Scarlet. He frequently blew up at her more vociferously than his barium chloride solutions! Colonel Mustard had been the interim assistant principal at the high school and was responsible for closing the campus when Miss Scarlet, then president of the student body, overthrew the administration and took control of the school.
You can obtain a sample of the murder weapon found at Miss Scarlet’s side by asking your teacher for it, in the CLASSROOM. If you can determine whether it is barium chloride or potassium iodate, 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? Write a lab report giving all evidence and analysis useful for the prosecution of the murderer. Your evidence will be Exhibit A at the trial and will be subjected to the scrutiny of the defense attorney. On the basis of your evidence alone, a murderer might be set free and an innocent person could be executed.
The Return of the White Lab Fiction Contest
Optional: Since many students feel compelled to launch into a barrage of verbiage describing their fictional exploits in this case as forensic scientists and detectives, I am awarding a 15 point bonus to the best written science fiction paper in the class. The paper should be:
- separate from your lab report.
- based upon the correct solution to the problem
- based directly or indirectly upon the episode of you, a forensic chemist, in pursuit of the murderer.
- funny enough to make your science teacher laugh at least once.
- considered the “best” by your science teacher.