CHEM 108 (past questions)

CHM 108: Chemistry Past Questions

Department of Chemical Sciences — 2024/2025 Session

Welcome to CHM 108

This page contains past questions from the CHM 108 chemistry practical course. Topics include separation techniques, chromatography, hydrocarbon properties, functional group identification, and salt analysis.

Course Coverage

• Fractional Crystallization
• Thin-Layer and Column Chromatography
• Physical & Chemical Properties of Hydrocarbons
• Functional Group Identification (Alcohols, Aldehydes, etc.)
• Qualitative Salt Analysis (Cations and Anions)

1. What is the primary goal of Practical 1?

• A. Synthesize salicylic acid
• B. Separate salicylic acid and CuSO₄·5H₂O
• C. Analyze gas solubility
• D. Purify ethanol

✅ Correct Answer: B

2. Fractional crystallization relies on differences in:

• A. Melting point
• B. Solubility
• C. Vapor pressure
• D. Density

✅ Correct Answer: B

3. Why is salicylic acid crystallized first in the experiment?

• A. It has higher solubility in water
• B. It is less soluble in cold, acidic water
• C. It reacts with CuSO₄
• D. It evaporates easily

✅ Correct Answer: B

4. What solvent is used to dissolve the mixture initially?

• A. Ethanol
• B. H₂SO₄ solution
• C. Distilled water
• D. Acetone

✅ Correct Answer: B

5. What is added to the CuSO₄ solution to induce crystallization?

• A. Water
• B. NaOH
• C. Ethanol
• D. HNO₃

✅ Correct Answer: C

6. What indicates complete crystallization of salicylic acid?

• A. Blue solution forms
• B. Needles settle at the bottom
• C. Solution boils
• D. Green precipitate forms

✅ Correct Answer: B

7. What apparatus is used for vacuum filtration?

• A. Erlenmeyer flask
• B. Buchner funnel
• C. Pipette
• D. Graduated cylinder

✅ Correct Answer: B

8. Why is the salicylic acid filter paper dried in an oven?

• A. To dissolve remaining CuSO₄
• B. To remove residual water
• C. To increase crystal size
• D. To change its color

✅ Correct Answer: B

9. What is the expected appearance of CuSO₄·5H₂O crystals?

• A. White needles
• B. Blue crystals
• C. Green powder
• D. Yellow precipitate

✅ Correct Answer: B

10. What error causes recovered mass to exceed initial mass?

• A. Used too much solvent
• B. Incomplete drying
• C. Added extra mixture
• D. Misread balance

✅ Correct Answer: B

11. How is the percentage of salicylic acid calculated?

• A. (Mass of mixture / Mass of salicylic acid) × 100
• B. (Mass of salicylic acid / Mass of mixture) × 100
• C. (Mass of CuSO₄ / Mass of mixture) × 100
• D. (Mass of salicylic acid / Mass of filter paper) × 100

✅ Correct Answer: B

12. Why is CuSO₄ more soluble in water than organic solvents?

• A. It is non-polar
• B. It forms hydrated ions
• C. It has a low melting point
• D. It is covalent

✅ Correct Answer: B

13. Why does salicylic acid have higher solubility in organic solvents?

• A. It is ionic
• B. It is non-polar
• C. It forms hydrogen bonds with water
• D. It reacts with water

✅ Correct Answer: B

14. What happens if CuSO₄ solution is not reduced to ~10 mL?

• A. No crystals form
• B. Salicylic acid precipitates
• C. Solution turns yellow
• D. Ethanol evaporates

✅ Correct Answer: A

15. What is used to rinse CuSO₄ crystals during filtration?

• A. Water
• B. Ethanol
• C. H₂SO₄
• D. NaOH

✅ Correct Answer: B

16. Why is the mixture heated on a water bath?

• A. To evaporate ethanol
• B. To dissolve solids
• C. To form precipitates
• D. To cool the solution

✅ Correct Answer: B

17. What should be avoided during heating?

• A. Stirring
• B. Boiling
• C. Cooling
• D. Filtering

✅ Correct Answer: B

18. What is the purpose of the ice bath in salicylic acid crystallization?

• A. Increase solubility
• B. Decrease solubility
• C. Change color
• D. Evaporate solvent

✅ Correct Answer: B

19. What is reported in the Practical 1 report sheet?

• A. Volume of ethanol
• B. Mass of beaker only
• C. Percent of components
• D. Temperature of water bath

✅ Correct Answer: C

20. What might cause CuSO₄·5H₂O crystals to weigh less initially?

• A. Excess water
• B. Ethanol contamination
• C. High temperature
• D. Small beaker

✅ Correct Answer: B. Ethanol contamination

21. What is the stationary phase in thin-layer chromatography (TLC)?

• A. Developing solvent
• B. Adsorbent on plate
• C. Plant extract
• D. Glass beaker

✅ Correct Answer: B. Adsorbent on plate

22. What is the mobile phase in TLC?

• A. Adsorbent layer
• B. Developing solvent
• C. Chloroplast extract
• D. Capillary tube

✅ Correct Answer: B. Developing solvent

23. What does the (R_f) value represent?

• A. Mass of compound
• B. Ratio of distances traveled
• C. Solubility in water
• D. Color of pigment

✅ Correct Answer: B. Ratio of distances traveled

24. How is (R_f) calculated?

• A. Distance by solvent / Distance by compound
• B. Distance by compound / Distance by solvent
• C. Mass of compound / Mass of solvent
• D. Distance by compound × Distance by solvent

✅ Correct Answer: B. Distance by compound / Distance by solvent

25. What is applied to the TLC slide to form a band?

• A. Solvent
• B. Chloroplast extract
• C. Adsorbent
• D. Water

✅ Correct Answer: B. Chloroplast extract

26. Why must the spotted band be above the solvent level?

• A. To dissolve the extract
• B. To prevent washing off
• C. To increase (R_f)
• D. To dry the slide

✅ Correct Answer: B. To prevent washing off

27. What covers the development chamber?

• A. Filter paper
• B. Foil paper
• C. Watch glass
• D. Plastic wrap

✅ Correct Answer: B. Foil paper

28. What indicates the solvent front in TLC?

• A. Pigment band
• B. Top of adsorbent
• C. Line marked after drying
• D. Bottom of slide

✅ Correct Answer: C. Line marked after drying

29. Why do some pigments move faster in TLC?

• A. Higher affinity for stationary phase
• B. Lower affinity for mobile phase
• C. Higher affinity for mobile phase
• D. Larger molecular weight

✅ Correct Answer: C. Higher affinity for mobile phase

30. What is the purpose of spotting multiple times?

• A. Increase solvent volume
• B. Form a concentrated band
• C. Change pigment color
• D. Dry the slide faster

✅ Correct Answer: B. Form a concentrated band

31. What is recorded in the Practical 2 report sheet?

• A. Mass of extract
• B. Colors and (R_f) values
• C. Volume of solvent
• D. Slide weight

✅ Correct Answer: B. Colors and (R_f) values

32. How is the chloroplast extract applied to the TLC slide?

• A. Pipette
• B. Capillary tube
• C. Brush
• D. Syringe

✅ Correct Answer: B. Capillary tube

33. What separates components in chromatography?

• A. Boiling point
• B. Affinity differences
• C. Density
• D. Melting point

✅ Correct Answer: B. Affinity differences

34. What is adsorption in chromatography?

• A. Absorption into solid
• B. Surface attachment
• C. Dissolving in solvent
• D. Evaporation

✅ Correct Answer: B. Surface attachment

35. How can TLC verify aspirin synthesis?

• A. Measure melting point
• B. Compare (R_f) with standard
• C. Test solubility
• D. Observe color change

✅ Correct Answer: B. Compare (R_f) with standard

36. What is the typical volume of developing solvent used?

• A. 1 mL
• B. 3 mL
• C. 5 mL
• D. 10 mL

✅ Correct Answer: B. 3 mL

37. Why is the TLC slide dried after development?

• A. To dissolve pigments
• B. To stop solvent movement
• C. To increase (R_f)
• D. To change colors

✅ Correct Answer: B. To stop solvent movement

38. What type of compounds are separated in Practical 2?

• A. Salts
• B. Chloroplast pigments
• C. Hydrocarbons
• D. Acids

✅ Correct Answer: B. Chloroplast pigments

39. What should be avoided when spotting the TLC slide?

• A. Drying between spots
• B. Touching the coating
• C. Using a capillary tube
• D. Marking scratches

✅ Correct Answer: B. Touching the coating

40. What is the role of capillary action in TLC?

• A. Dissolves extract
• B. Moves solvent up the plate
• C. Forms pigments
• D. Dries the slide

✅ Correct Answer: B. Moves solvent up the plate

41. What is the purpose of Practical 5?

• A. Synthesize organic compounds
• B. Identify functional groups
• C. Measure boiling points
• D. Purify solvents

✅ Correct Answer: B. Identify functional groups

42. Which compound is tested for solubility in Practical 5?

• A. Hexane
• B. Benzoic acid
• C. Toluene
• D. Kerosene

✅ Correct Answer: B. Benzoic acid

43. What indicates a compound is soluble in water?

• A. Cloudy solution
• B. Transparent solution
• C. Precipitate forms
• D. Color change

✅ Correct Answer: B. Transparent solution

44. What pH is expected for diethylamine in water?

• A. Acidic
• B. Neutral
• C. Basic
• D. No pH change

✅ Correct Answer: C. Basic

45. Which test identifies alcohols?

• A. Silver nitrate
• B. Ceric nitrate
• C. 2,4-DNPH
• D. Bromination

✅ Correct Answer: B. Ceric nitrate

46. What color change indicates a positive ceric nitrate test?

• A. Red to yellow
• B. Yellow to red
• C. Blue to green
• D. Colorless to blue

✅ Correct Answer: B. Yellow to red

47. What forms in a positive silver nitrate test for alkyl halides?

• A. Silver mirror
• B. Red precipitate
• C. Silver halide precipitate
• D. Yellow solution

✅ Correct Answer: C. Silver halide precipitate

48. What does a positive 2,4-DNPH test indicate?

• A. Alcohol
• B. Alkene
• C. Aldehyde/ketone
• D. Amine

✅ Correct Answer: C. Aldehyde/ketone

49. Which compound gives a positive Tollens test?

• A. Acetone
• B. Isobutyl aldehyde
• C. Ethanol
• D. Hexane

✅ Correct Answer: B. Isobutyl aldehyde

50. What is observed in a positive Tollens test?

• A. Yellow precipitate
• B. Silver mirror
• C. Red color
• D. Gas bubbles

✅ Correct Answer: B. Silver mirror

51. What does a positive bromination test indicate?

• A. Alkane
• B. Alkene
• C. Aldehyde
• D. Carboxylic acid

✅ Correct Answer: B. Alkene

52. What happens to bromine color in a positive bromination test?

• A. Turns blue
• B. Decolorizes
• C. Turns green
• D. Intensifies red

✅ Correct Answer: B. Decolorizes

53. Which solvent is used in the bromination test?

• A. Water
• B. Cyclohexane
• C. Ethanol
• D. NaOH

✅ Correct Answer: B. Cyclohexane

54. What is tested if a compound is insoluble in water?

• A. Ceric nitrate
• B. 6 M NaOH
• C. Tollens
• D. Bromination

✅ Correct Answer: B. 6 M NaOH

55. Which compound is soluble in 6 M NaOH?

• A. Acetone
• B. Diethylamine
• C. Benzoic acid
• D. Ethanol

✅ Correct Answer: C. Benzoic acid

56. What is the pH of acetone in water?

• A. Acidic
• B. Basic
• C. Neutral
• D. Variable

✅ Correct Answer: C. Neutral

57. Why is Tollens reagent prepared fresh?

• A. It evaporates quickly
• B. It is unstable
• C. It changes color
• D. It is toxic

✅ Correct Answer: B. It is unstable

58. What is added to prepare Tollens reagent?

• A. Ethanol
• B. Ammonia
• C. HNO₃
• D. NaCl

✅ Correct Answer: B. Ammonia

59. Which test requires heating in a water bath?

• A. Ceric nitrate
• B. Silver nitrate
• C. Tollens
• D. Bromination

✅ Correct Answer: C. Tollens

60. What should be done with Tollens test waste?

• A. Pour down the drain
• B. Dispose in nitric acid
• C. Store in a bottle
• D. Evaporate

✅ Correct Answer: B. Dispose in nitric acid