Chemistry 161 AND Chemistry 261

SAMPLE LAB EXAM QUESTIONS

(Questions taken from previous lab exams)

Time: 90 minutes

Answer each question directly on the test paper by circling the appropriate letter. The mark assigned to each question is shown in brackets. There is only one correct answer for each question. Marks will be deducted for multiple answers.

Atomic weights: Br, 79.9; C, 12; Ca, 40; Cl, 35.5; H, 1; O,16; ; Na, 23; S, 32.

INFRARED SPECTROSCOPY: CORRELATION CHART

Table of characteristic frequencies

Type of vibration

Frequency of absorption band (cm-1)

Intensity

C - H

alkanes (stretch)

300 - 2850

strong

 

alkenes (stretch)

3100 - 3000

medium

 

aromatics (stretch)

3150 - 3050

weak

 

alkyne (stretch)

ca. 3300

strong

 

aldehyde (stretch)

2900 - 2800 and

2800 - 2700

weak

       

C = C

alkene

1680 - 1600

medium - weak

 

aromatic

1600 - 1400

medium - weak

alkyne

2250 - 2100

medium - weak

       

C = O

aldehyde

1740 - 1720

strong

 

ketone (acyclic)

1725 - 1705

strong

 

carboxylic acid

1725 - 1700

strong

 

ester

1750 - 1730

strong

 

amide

1700 - 1640

strong

 

anhydride

ca. 1800 and 1760

strong

       

C - O

alcohols, ethers, esters, acids

1300 - 1000

strong

(sometimes absent)

       

O - H

alcohols / phenols

   
 

free

3650 - 3600

medium

 

hydrogen bonded

3400 - 3200

medium

 

carboxylic acid

3300 - 2800

medium

       

N - H

primary

3500 and 3400

medium

 

secondary

ca 3450

medium

       

nitriles

2260 - 2240

medium

       

N = O

nitro (R-NO 2)

1600 - 1500 and

1400 - 1300

strong

 

  1. Gallstones consist very largely of cholesterol (1). The crude cholesterol can be isolated by heating the gallstones with dioxane, then adding methanol and decolorizing charcoal, filtering, and cooling the solution.

    Crude cholesterol is purified by bromination (reactions 1) and debromination (reaction 2).

  1. Solids are very often purified by recrystallization from an appropriate solvent. Why is it not possible to purify the cholesterol isolated from gallstones in this way? (1 point)
    1. The cholesterol would decompose on heating in a solvent
    2. The impurities are similar in structure and would co-precipitate
    3. The melting point of cholesterol is too high
    4. The proportion of impurities present is too small
    5. It is not possible to find an appropriate solvent
    6. Cholesterol is too large a molecule to recrystallize successfully
  2.  

  3. How is the crude cholesterol (isolated directly from gallstones) separated from the impurities by being converted to the dibromide? (1 point)
    1. The impurities stay in solution
    2. The impurities are removed as a solid
    3. The impurities precipitate
    4. The cholesterol dibromide stays in solution
    5. The impurities are converted to cholesterol dibromide
    6. The impurities are converted to an insoluble precipitate
  4.  

  5. What reagent is used in the laboratory to brominate cholesterol (reaction 1)? (1 point)
    1. Bromine in carbon tetrachloride
    2. Bromine and sodium acetate in acetic acid
    3. Bromine in aqueous solution
    4. Bromine and potassium bromide in water
    5. Bromine in dilute sodium hydroxide
    6. Pure bromine
  6.  

  7. What reagent is used in the laboratory to debrominate cholesterol (reaction 2)? (1 point)
    1. Sodium hydroxide
    2. Aqueous potassium permanganate
    3. Concentrated sulfuric acid
    4. Zinc dust and acetic acid
    5. Hydrochloric acid
    6. Copper powder
  8.  

  9. Why is the ethereal solution of cholesterol after debromination washed with 10% sodium hydroxide solution? (1 point)
    1. To remove the bulk of the water
    2. To remove traces of acid
    3. To remove traces of base
    4. To dissolve the ether
    5. To extract the cholesterol
    6. To precipitate the cholesterol
  10.  

  11. Why is the ethereal solution of cholesterol after debromination washed with saturated sodium chloride solution? (1 point)
    1. To remove the bulk of the water
    2. To remove traces of acid
    3. To remove traces of base
    4. To dissolve the ether
    5. To extract the cholesterol
    6. To precipitate the cholesterol
  12.  

  13. The final purification of cholesterol is recrystallization from the two solvent system of ether - methanol. Why can the cholesterol not be recrystallized from ether alone? (1 point)
    1. The product will not be pure
    2. Ether is too low-boiling solvent
    3. It will not form well-shaped crystals
    4. It is not sufficiently soluble in ether
    5. It is too soluble in ether
    6. It forms a complex with ether
  14.  

  15. When recrystallizing a solid using a two-solvent system, what characteristic should the two solvents have? (1 point)
    1. They should both be high boiling
    2. They should be immiscible with one another
    3. The solid should be insoluble in both solvents
    4. The solid should be very soluble in one solvent and insoluble in the other solvent
    5. The solid should be very soluble in both solvents
    6. The solvents should have the same degree of solubility in both solvents
  1. A thin layer chromatographic analysis on silica gel of the crude cholesterol isolated from gallstones(chromatogram A) and purified cholesterol (chromatogram B) are shown below.

  1. The thin layer chromatogram "A" indicates the following about the extract. (1 point)
    1. It is a crystalline compound
    2. It is a mixture of at least five components
    3. It is a mixture of more than seven components
    4. It is a pure compound
    5. It is a mixture of four components
    6. It melts with a sharp melting range
  2.  

  3. Given that the TLC of pure cholesterol is as shown in chromatogram B above, what is the calculated Rf value of cholesterol? (1 point)
    1. 0.29
    2. 3.35
    3. 0.38
    4. 1.00
    5. 0.23
    6. 2.59
  4.  

  5. Given that the TLC of pure cholesterol is as shown in chromatogram B above, what would be the predicted Rf of cholesterol if the TLC of cholesterol was run in a less polar solvent system, e.g., hexane/ethyl acetate, 5/l. (1 point)
    1. The spot for cholesterol would be at the solvent front, i.e., Rf~1
    2. The spot for cholesterol would be at the origin i.e., Rf~0
    3. The Rf would be change slightly
    4. The Rf would be a larger number
    5. The Rf would be a smaller number
    6. The Rf would remain unchanged
  6.  

  7. Given that the TLC of pure cholesterol is as shown in chromatogram B above, what would be the predicted Rf of cholesterol if the TLC of cholesterol was run in a more polar solvent system, e.g., hexane / ethyl acetate, 1/2. (1 point)
    1. The Rf would be a larger number
    2. The spot for cholesterol would be at the solvent front, i.e., Rf~1
    3. The Rf would be about the same
    4. The Rf would be a smaller number
    5. The Rf would remain unchanged
    6. The spot for cholesterol alcohol would be at the origin i.e., Rf~0
  1. Lidocaine sulfate (2), often sold under the trade name of Xylocaine, is one of the local anesthetics most commonly used in dentistry. It is a colorless, crystalline material which melts at 210 - 212°C when recrystallized from acetone. Compound 2 was synthesized in the laboratory. The initial product had a melting point of 201 - 204°C, was orange-colored, and contained black specks. Therefore, it was necessary to purify it by recrystallization.

  1. What is the initial step in the recrystallization of about 1 g of lidocaine sulfate? (1 point)
    1. Dissolving the solid in the minimum volume of cold acetone
    2. Dissolving the solid in the minimum volume of boiling acetone
    3. Dissolving the solid in the minimum volume of boiling water
    4. Heating the solid with 100 mL of acetone under reflux
    5. Dissolving the solid in the maximum volume of boiling acetone
    6. Collecting the solid by vacuum filtration
  2.  

  3. What equipment would you require for the first step in the recrystallization of 1 g of lidocaine sulfate? (1 point)
    1. Erlenmeyer flask, Bünsen burner, boiling stones
    2. Büchner funnel, side arm flask, steam bath, filter paper
    3. Condenser, Erlenmeyer flask, Bünsen burner, boiling stones
    4. Filter funnel, two Erlenmeyer flasks, filter paper
    5. Steam bath, two Erlenmeyer flasks, boiling stones
    6. Ice bath, thermometer, beaker, stirring rod
  4.  

  5. During the recrystallization process, some black insoluble material remained in the hot saturated solution. What action would you take? (1 point)
    1. Gravity filter through fluted filter paper held in a heated filter funnel
    2. Vacuum filter through a Büchner funnel
    3. Add cold solvent and reboil the solution
    4. Gravity filter through a Büchner funnel
    5. Add boiling solvent to dissolve the insoluble material
    6. Add charcoal, boil and gravity filter
  6.  

  7. On standing at room temperature, crystals formed in the solution. What should be done next? (1 point)
    1. Collect the crystals by gravity filtration
    2. Warm the solution to reduce the volume of the solvent
    3. Collect the crystals by vacuum filtration
    4. Cool the solution in an ice bath
    5. Pour the solution into a beaker and allow the solvent to evaporate
    6. Allow the solution to stand at room temperature for 15 minutes
  8.  

  9. What equipment should be used to collect the crystals after recrystallization? (1 point)
    1. Filter funnel, Erlenmeyer flask, filter paper
    2. Filter funnel, side-arm flask, trap, filter paper
    3. Filter funnel, beaker, ring clamp, filter paper
    4. Büchner funnel, Erlenmeyer flask, filter paper
    5. Büchner funnel, Erlenmeyer flask, trap, filter paper
    6. Büchner funnel, side-arm flask, trap, filter paper
  10.  

  11. When the crystals were collected, they were slightly orange-colored. What should be tried? (1 point)
    1. Wash the crystals with a small quantity of cold water
    2. Wash the crystals with the solvent from which they recrystallized
    3. Add charcoal to the crystals
    4. Wash the crystals with a small quantity of warm water
    5. Wash the crystals with a small quantity of cold acetone
    6. Wash the crystals with a small quantity of warm acetone
  12.  

  13. The recrystallized lidocaine sulfate was white well-shaped crystals. The melting point was 196-197°C. What is the most likely reason for this? (1 point)
    1. Impurities had been introduced during recrystallization
    2. The crystals contained water
    3. The crystals contained acetone
    4. The crystals were contaminated with charcoal
    5. The crystals had decomposed during recrystallization
    6. The crystals were contaminated with fibres from the filter paper
  14.  

  15. The yield of recrystallized material from 1.15 g of lidocaine sulfate was 0.73 g. What percentage of the product was recovered? (1 point)
    1. 36.5%
    2. 51.3%
    3. 68.7%
    4. 71.2%
    5. 63.5%
    6. 79.1%
  1. Cyclohexyl bromide (3) can be prepared from cyclohexanol (4) by reaction with sodium bromide in the presence of an acid catalysts. In one such preparation, water (20 mL), sodium bromide (20 g) and cyclohexanol (10.5 mL, density 0.96) were mixed and concentrated sulfuric acid (20 mL; density 1.85) was added dropwise to the swirled solution over about two minutes. The mixture was heated under reflux for five minutes, then distilled until no more water-insoluble droplets came over. The distillate was washed with water (10 mL), ice-cold concentrated sulfuric acid (10 mL), 10% sodium hydroxide solution (10 mL, density 1.0), and water (10 mL). Calcium chloride was added to the product. After filtration, the liquid was distilled and the fraction boiling between 155 and 165°C was collected. The yield was 8.1 g.  This specific experiment was not studied during Fall Session 1998.  Pay attention to example yield calculation.

  1. Why is the sulfuric acid added to the aqueous mixture and not the other way around? (1 point)
    1. Two layers will form if an aqueous solution is added to sulfuric acid.
    2. The product will be different if the aqueous mixture is added to sulfuric acid.
    3. The liquid will freeze and solidify if the aqueous mixture is added to sulfuric acid.
    4. The solution will overheat, splatter, and perhaps explode if the aqueous mixture is added to sulfuric acid.
    5. The solution will immediately catch fire if the aqueous mixture is added to sulfuric acid.
    6. The solution will become thick and syrupy if the aqueous mixture is added to sulfuric acid.
  2.  

  3. What pieces of glass equipment would be used to heat the mixture under reflux? (1 point)
    1. Round bottom flask, condenser
    2. Erlenmeyer flask, condenser, thermometer
    3. Beaker, Erlenmeyer flask, separatory funnel
    4. Round bottom flask, three-way connecting tube, condenser
    5. Two round bottom flask, three-way connecting tube, condenser, bent adapter with vacuum take-off, thermometer
    6. Round bottom flask, condenser, thermometer
  4.  

  5. There are two layers in the distillate. What would they be? (1 point)
    1. Cyclohexanol and water
    2. Cyclohexanol and cyclohexyl bromide
    3. Cyclohexanol and sodium bromide
    4. Cyclohexyl bromide and concentrated sulfuric acid
    5. Cyclohexyl bromide and water
    6. Cyclohexanol and concentrated sulfuric acid
  6.  

  7. Why was the product washed with 10% sodium hydroxide solution? (1 point)
    1. To remove the water
    2. To complete the reaction
    3. To react with the remaining sodium bromide
    4. To change the product from a salt to free cyclohexyl bromide
    5. To remove cyclohexanol
    6. To remove sulfuric acid
  8.  

  9. What apparatus should be used to wash the product? (1 point)
    1. Beaker
    2. Erlenmeyer flask
    3. Separatory funnel
    4. Test tube
    5. Side arm flask
    6. Round bottom flask
  10.  

  11. What is the limiting reagent in the reaction of cyclohexanol to cyclohexyl bromide? (2 points)
    1. Sodium bromide
    2. Cyclohexanol
    3. Concentrated sulfuric acid
    4. Water
    5. 10% Sodium hydroxide solution
    6. Calcium chloride
  12.  

  13. Why was calcium chloride added to the product? (1 point)
    1. To remove water
    2. To remove cyclohexanol
    3. To remove sulfuric acid
    4. To remove water and cyclohexanol
    5. To remove cyclohexanol and sodium bromide
    6. To remove water and sodium hydroxide
  14.  

  15. What is the theoretical yield of cyclohexyl bromide from the quantities of reagents used? (2 points)
    1. 4.1 g
    2. 8.2 g
    3. 16.4 g   Wt = mL x d = 10.5 x 0.96 = 10.1 g
    4. 17.1 g Theoretical yield = 10.1 x 163.1/100.2 = 16.4 g
    5. 31.7 g
    6. 61.5 g
  16.  

  17. What is the percentage yield of cyclohexyl bromide? (2 points)
    1. 197.6%
    2. 98.8%
    3. 49.3% 8.1 x 100/16.4 = 49.3%
    4. 47.4%
    5. 25.6%
    6. 13.2%
  18.  

  19. What action should be taken if sulfuric acid is splattered on the skin? (1 point)
    1. Wipe the skin with a towel
    2. Wash the skin with 10% sodium hydroxide solution
    3. Grin and bear it
    4. Drench the skin with water for at least 10 minutes
    5. Cover the area with a gauze bandage or a bandaid
    6. Apply Ozonol or other ointment
  20.  

  21. WHMIS, the "Workers Hazardous Materials Information System" uses standardized symbols on the labels of compounds to indicate the potential hazard, if any, associated with the particular type of compound. Which of the WHMIS symbols shown below is associated with the hazard of sulfuric acid (1 point)
  1. A technician working in the undergraduate Chemistry X61 laboratory found the labels had fallen off five bottles of chemicals. He performed a simple solubility test or ran an infrared spectrum of the contents of each of the bottles. Help him match the correct structure from those given below with the pertinent chemical fact or infrared spectrum so that the correct label can be put on each container. An Infrared Correlation Chart is included on the previous page of this test paper.

  1. This compound is only slightly soluble in water, but is soluble in dilute sodium hydroxide solution and in saturated potassium bicarbonate. What is the most likely structure of the compounds A to F for the component in bottle 1? (2 points)
    1. Benzaldehyde
    2. Benzyl amine
    3. Benzyl alcohol
    4. Benzonitrile
    5. Benzoic acid
    6. 2,5-Dimethylphenol
  2.  

  3. The infrared spectrum of the contents of bottle 2 is shown below. What is the most likely structure of the compounds A - F for the component in bottle 2? (2 points)

    1. Benzaldehyde
    2. Benzyl amine
    3. Benzyl alcohol
    4. Benzonitrile
    5. Benzoic acid
    6. 2,5-Dimethylphenol
  4.  

  5. The infrared spectrum of the contents of bottle 3 is shown below. What is the most likely structure of the compounds A - F for the component in bottle 3? (2 points)

    1. Benzaldehyde
    2. Benzyl amine
    3. Benzyl alcohol
    4. Benzonitrile
    5. Benzoic acid
    6. 2,5-Dimethylphenol
  6.  

  7. The infrared spectrum of the components of bottle 4 is shown below. What is the most likely structure of the compounds A - F for the component in bottle 4? (2 points)

    1. Benzaldehyde
    2. Benzyl amine
    3. Benzyl alcohol
    4. Benzonitrile
    5. Benzoic acid
    6. 2,5-Dimethylphenol
  8.  

  9. The contents of bottle 5 appear to be the same as one of the bottles 1-4 described above since the component of bottle 5 is insoluble in water and dilute base, but is soluble in dilute acid. What is the most likely structure of the compounds A - F for the component in bottle 5? (2 points)
    1. Benzaldehyde
    2. Benzyl amine
    3. Benzyl alcohol
    4. Benzonitrile
    5. Benzoic acid
    6. 2,5-Dimethylphenol
  10.  

  11. The compounds A - F shown above may be classified according to their solubilities. Which of the following groups of compounds is insoluble in aqueous acid and aqueous base, but soluble in dichloromethane? (2 points)
    1. Benzaldehyde, benzyl amine, and benzyl alcohol
    2. Benzyl amine, benzonitrile, and benzoic acid
    3. Benzyl alcohol, benzaldehyde, and benzonitrile
    4. Benzonitrile, benzoic acid, and 2,5-dimethylphenol
    5. Benzoic acid, benzaldehyde, and benzyl amine
    6. 2,5-Dimethylphenol, benzoic acid, and benzyl alcohol
  1. Several undergraduate students in the Chemistry X61 laboratory were given a sample of a compound which was known to be either malic acid (5) or citric acid (6). When authentic samples of the known compounds are available, the physical property of melting point or Rf may be used to establish the identity of the compound.

  1. One student measured the melting point of the unknown carboxylic acid and found it to be 99 - 100oC. The melting point of pure citric acid and pure malic acid are reported as 100 oC and 98 - 100 oC, respectively. What would you advise this student to do to further identify the compound? (1 point)
    1. Calibrate the thermometer, and on remeasuring the melting point as 99 - 100oC, conclude that the unknown sample contains citric acid
    2. Measure the melting point of an authentic sample of malic acid and on observing it to be 99 - 100oC, conclude that the unknown sample is malic acid.
    3. Take a mixture melting point of the unknown sample with citric acid and on observing it to be 78 - 84oC, conclude that the unknown sample is citric acid
    4. Take a mixture melting point of the unknown sample with malic acid and on observing it to be 98 - 100oC, conclude that the unknown sample is malic acid
    5. Check the literature for the reported melting points of citric acid and malic acid
    6. Recrystallize the unknown sample, dry the crystals, and measure the melting point as 99 - 100oC, concluding that the unknown sample is citric acid
  2.  

  3. A second student examined the sample, citric acid, and malic acid by thin layer chromatography. When the solvent front had moved 5.0 cm. above the level of the original spots, the sample showed an Rf of 0.7 while malic acid was 3.5 cm and that of citric acid was 4.0 cm above the original spot. Which of the following statements is true? (1 point)
    1. The Rf (retardation factor) is defined as the distance the sample travelled over the distance the solvent travelled
    2. The Rf value of citric acid is 0.57
    3. Rf is an abbreviation of resolution frequency
    4. The Rf value of malic acid is 1.42
    5. The Rf (retardation factor) is defined as the distance the solvent travelled over the distance the sample travelled
    6. The Rf value of citric acid is 1.25

       

  4.  

  5. Various polyamide polymers are prepared by reaction of a dicarboxylic acid (or derivative) with a diamine. Nylon 6-6 and Nylon 6-10 are prepared for industrial use in this way. Which of the following statements is true about the preparation of nylon? (1 point) (Polymerization not studied during Fall Session, 1998!)

    1. The designation 6-6 or 6-10 comes from the ratio of diamine to dicarboxylic acid
    2. Both reagents are mixed in one phase, then a second phase is added and the polymer transfers to the second phase
    3. Polyamide polymers are classified as addition polymers because during polymerization, hydrogen chloride is generated
    4. Polymerization occurs at the interface of two phases, each of which contains one of the reactants, and the product is obtained by removal of a thin film which is formed.
    5. Polymerization is initially carried out by carefully mixing the reagents and is completed by heating
    6. All of the above
  6.  

  7. Lucite, also known as Plexiglas, is an addition polymer made from the monomer methyl methacrylate. Which of the following statements is true about the preparation of Lucite? (1 point)  (Polymerization not studied during Fall Session, 1998!)

    1. Methyl methacrylate is a derivative of acrylic acid
    2. Methyl methacrylate polymers are also known as polyacrylics
    3. In this type of polymerization, the monomer units simply add to one another without elimination of a small molecule
    4. Plexiglas is prepared by a free radical polymerization
    5. Benzoyl peroxide is the free radical source which initiates polymerization of methyl methacrylate
    6. All of the above

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Updated July 24, 2001