LAB 7 - IDENTIFICATION OF AN UNKNOWN COMPOUND
- Page ID
- 506284
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The purpose of this experiment is to:
- Determine the boiling point and density of an unknown organic compound.
- Test the pH and solubility of an unknown organic compound.
- Perform chemical tests on an unknown organic compound.
- Identify an unknown organic compound based on chemical tests and physical properties.
INTRODUCTION
Over the past several experiments, you explored physical properties, structure, reactions, and chemical tests of organic compounds containing various functional groups. Now, you will use these methods to identify an unknown compound. Each lab group will be given a unique unknown. You will first test the pH of the unknown and then determine whether it is soluble in various solvents. Next, you will determine the boiling point and density of your compound. Finally, you will perform six chemical tests on the unknown. Using your results and various molecular formulas, you will propose a structure for your unknown compound.
1) Wear chemical splash goggles while working on this experiment.
2) Handle all reagents and perform all parts of this experiment under a working fume hood.
3) Gloves are provided, and we encourage you to wear them.
4) Be careful while handling hot glassware after running the simple distillation.
5) Dispose of all waste in the designated container and clean all equipment and your work area when you are finished.
CHEMICALS AND EQUIPMENT NEEDED
| CHEMICALS | CHEMICALS | EQUIPMENT | EQUIPMENT |
|---|---|---|---|
| Unknown | Bromine test solution | 10 small or medium test tubes | Hot plate |
| Hexane | Potassium permanganate test solution | Test tube holder | 400. mL beaker |
| Ethanol | Chromic acid test solution | Simple distillation apparatus and equipment | Thermometer |
| Methylene chloride | Ferric chloride test solution | 10 mL graduated cylinder | pH paper |
| Iodoform test solution | Benedict’s test solution | Disposable droppers | Stirring rod |
| Water | 10% NaOH |
Halogenated organic waste containers |
Non-halogenated organic waste containers |
*Note: Refer to previous labs for concentration and preparation of test solutions.* (The whole lab shares test solutions)
EXPERIMENTAL PROCEDURE
Part A: pH and Solubility of the Unknown
1) Gather four test tubes and a test tube rack. Add 10 drops of your unknown to each test. Dip a stirring rod into one test tube and touch the tip of the rod to a strip of pH paper. Record the pH of your unknown.
2) Under the fume hood, add 10 drops of water, hexane, ethanol, and methylene chloride (using only one type of solvent in each test tube) to the four test tubes with the unknown from step one. Briefly shake each test tube and assess the solubility of the unknown substance in the provided solvent. Dispose of the contents of the test tubes in the halogenated waste container. Clean all equipment before proceeding to the next step.
Part B: Boiling Point of the Unknown
1) Add 10 mL of your unknown to a round-bottom flask and set up a simple distillation apparatus. After your instructor approves the setup, use simple distillation to determine the boiling point of your unknown. Remember, never distill to dryness.
2) Disassemble the simple distillation apparatus and clean all equipment before proceeding to the next step.
3) Dispose of the unknown, as directed by your instructor.
Part C: Density of the Unknown.
1) Determine the mass of an empty 10.0 mL graduated cylinder.
2) Add 5.0 mL to 10.0 mL of the unknown to the graduated cylinder and record the correct volume.
3) Record the combined mass of the unknown and graduated cylinder.
4) Subtract the mass of the empty graduated cylinder from the combined mass of the unknown and cylinder to obtain the mass of the unknown.
5) Using the unknown's mass and the unknown's volume (from step 2), calculate the density of the unknown, rounding your answer to the appropriate number of significant digits.
6) Repeat steps 1-5 two more times using a different amount of the unknown.
7) Calculate the average density of the unknown by adding up the three density values and dividing the sum by three.
8) Dispose of the unknown substance as directed by your instructor and clean all equipment before proceeding to the next step.
Part D: Chemical Tests (Remember to work under the fume hood)
1) Obtain six test tubes and add 10 drops of your unknown to each.
2) To test tube 1, add 3-5 drops of the bromine test reagent. Gently swirl the contents of the test tube and record your observations.
3) To test tube 2, add 3-5 drops of the potassium permanganate test reagent. Gently swirl the contents of the test tube and record your observations.
4) To test tube 3, add 3-5 drops of the chromic acid test reagent. Gently swirl the contents of the test tube and record your observations.
5) To test tube 4, add 3-5 drops of the ferric chloride test reagent. Gently swirl the contents of the test tube and record your observations.
6) To test tube 5, add 2 mL of water and 10 drops of 10% NaOH. Obtain a hot plate and a 400 mL beaker, half-filled with water. Place the test tube in the beaker with water and set the beaker on a hot plate. Warm the water bath to 55 °C. While keeping the test tube in the water bath, add 20 drops of the iodoform reagent. Mix using a glass stirring rod. After 2 minutes, record your observations.
7) To test tube 6, add 2 mL of Benedict's reagent. Mix the contents using a glass stirring rod, and then place the test tubes in a 400 mL beaker that is half-filled with water. Heat it to a boil. Once the water starts to boil, let the test tube sit in the water bath for 5 minutes. Record your observations.
8) Discard the contents of each test tube as directed by your instructor.
PRE-LAB QUESTIONS
Name ____________________________________
1) Briefly describe the purpose of the following chemical tests. Explain what both positive and negative test results would look like.
Bromine Test:
Potassium Permanganate Test:
Chromic Acid Test:
Ferric Chloride Test:
Benedict’s Test:
Iodoform Test:
2) Which of the above chemical tests can distinguish between the following pairs of compounds?
Part A: 2-pentanone and 3-pentanone
Part B: Cyclohexane and Cyclohexane
Part C: 2-butanol and 2-methyl-2-butanol
Part D: Toluene and Phenol
Part E: Hexanal and 2-Hexanone
3) Draw three constitutional isomers with molecular formula C3H6O. Identify any non-alkane functional groups present in the compounds drawn. Explain which chemical tests can be used to identify the compounds drawn.
DATA AND OBSERVATIONS
Name _________________________Lab Partner(s) ______________________________
Unknown Letter or Number =
Part A: pH and Solubility of the Unknown
pH of unknown =
Solubility of the Unknown
Solvent |
Yes/No |
|---|---|
|
Water |
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Hexane |
|
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Ethanol |
|
|
Methylene chloride |
What do the results of pH and solubility tests tell you about your unknown?
Part B: Boiling Point of the Unknown
Boiling point of unknown (from simple distillation) =
Part C: Density of the Unknown
Trial 1 |
Trial 2 |
Trial 3 |
|
|---|---|---|---|
|
Mass of an empty 10 mL graduated cylinder |
|||
|
Mass of the graduated cylinder and the unknown |
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Mass of unknown |
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Volume of unknown |
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|
Density of unknown |
Show calculations for the density of each trial and the average density.
Part D: Chemical Tests (Remember to Work Under the Fume Hood)
Test |
Functional Group Tested for |
Observations |
Positive or Negative Test |
|---|---|---|---|
|
Bromine Test |
|||
|
Potassium Permanganate Test |
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Chromic Acid Test |
|||
|
Ferric Chloride Test |
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Benedict’s Test |
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|
Iodoform Test |
Briefly summarize what the chemical tests infer about your unknown.
POST-LAB QUESTIONS
1) Your unknown has one of the following molecular formulas:
C3H8O, C4H8O, C6H6O, C3H6O2, C4H10O
Based on the results from part D (chemical tests), propose a structure for your unknown. Be sure to explain your decision in detail.
2) Consider the structure drawn in post-lab question 1. Perform a literature search to determine the compound's actual density and boiling point. Does this information agree with your unknown choice? Explain.
3) Does the result of the pH test support your unknown decision? Explain.
4) Based on answers provided for post lab questions 1-3, how confident are you in your decision about the unknown’s identity? Explain.
5) Can the unknown positively be identified based solely on the results of this experiment? If not, what other tests would need to be performed?
Please click here to access the Pre-Lab, Data Tables, and Post-Lab in Word or PDF format. Complete them and upload the lab report according to your instructor's instructions.