Structure and uses of acetic acid MCQs With Answer

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Introduction

Acetic acid is a fundamental carboxylic acid widely encountered in pharmaceutical science. This concise overview explains the structure of acetic acid (CH3COOH), resonance stabilization, acidity (pKa ≈ 4.76), and key chemical behaviors such as esterification, acetylation, and formation of acetate salts. Practical pharmaceutical uses include solvent, preservative, intermediate in drug synthesis, buffer component, and reagent in formulation and quality control. Analytical identification (IR, NMR, titration) and safe handling are essential topics for B. Pharm students. Keywords: acetic acid, structure of acetic acid, uses of acetic acid, acetate, carboxylic acid, B. Pharm MCQs. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which structural feature defines acetic acid?

  • A methyl group attached to a hydroxyl group
  • A benzene ring with a carboxyl substituent
  • A methyl group attached to a carboxyl group
  • An ethyl group attached to an aldehyde

Correct Answer: A methyl group attached to a carboxyl group

Q2. What is the molecular formula of acetic acid?

  • CH4O
  • CH3COOH
  • C2H6O
  • C2H4O2

Correct Answer: CH3COOH

Q3. What is the approximate pKa value of acetic acid?

  • 0.0
  • 2.5
  • 4.76
  • 7.4

Correct Answer: 4.76

Q4. Which resonance contributor explains acetic acid’s acidity?

  • Delocalization of the methyl hydrogen
  • Delocalization of the lone pair electrons on oxygen into the carbonyl
  • Delocalization of electrons over the methyl group
  • Conjugation with an aromatic ring

Correct Answer: Delocalization of the lone pair electrons on oxygen into the carbonyl

Q5. Which IR absorption is characteristic of the C=O stretch in acetic acid?

  • ~3300 cm⁻¹
  • ~1700 cm⁻¹
  • ~2100 cm⁻¹
  • ~1500 cm⁻¹

Correct Answer: ~1700 cm⁻¹

Q6. In 1H NMR, the methyl group (CH3) of acetic acid typically appears around which chemical shift?

  • 0.5 ppm
  • 2.0–2.5 ppm
  • 5.0–6.0 ppm
  • 10–12 ppm

Correct Answer: 2.0–2.5 ppm

Q7. Which term describes nearly pure acetic acid used in labs and industry?

  • Diluted acetic acid
  • Concentrated acetic acid
  • Glacial acetic acid
  • Absolute acetic acid

Correct Answer: Glacial acetic acid

Q8. Industrially, which process is most commonly used to produce acetic acid?

  • Synthetic fermentation of ethanol only
  • Methanol carbonylation
  • Direct oxidation of ethane
  • Hydration of acetylene

Correct Answer: Methanol carbonylation

Q9. Which reagent converts acetic acid to acetic anhydride in the laboratory?

  • Sodium hydroxide
  • Phosphorus pentoxide (dehydrating agent)
  • Hydrogen peroxide
  • Sodium borohydride

Correct Answer: Phosphorus pentoxide (dehydrating agent)

Q10. In Fischer esterification, acetic acid reacts with an alcohol to form what product?

  • Acyl chloride
  • Ester
  • Amide
  • Ether

Correct Answer: Ester

Q11. Which salt is formed when acetic acid neutralizes sodium hydroxide?

  • Sodium acetate
  • Potassium acetate
  • Calcium acetate
  • Sodium bicarbonate

Correct Answer: Sodium acetate

Q12. Acetylation of a drug molecule commonly uses which acetic derivative?

  • Acetic acid
  • Acetic anhydride
  • Acetone
  • Sodium acetate

Correct Answer: Acetic anhydride

Q13. Which property makes acetic acid a useful solvent in pharmaceutical formulations?

  • High volatility and lack of polarity
  • Moderate polarity and miscibility with water and organic solvents
  • Strong basicity
  • Inertness with all reagents

Correct Answer: Moderate polarity and miscibility with water and organic solvents

Q14. Which of the following is a common pharmaceutical use of acetic acid?

  • Active ingredient in antihypertensives
  • Topical antiseptic and preservative
  • Antiviral agent
  • Direct anaesthetic

Correct Answer: Topical antiseptic and preservative

Q15. Which analytical method is most useful for detecting the carboxylic O–H stretch of acetic acid?

  • UV-Visible spectroscopy
  • Infrared spectroscopy
  • Mass spectrometry
  • Gas chromatography with flame ionization

Correct Answer: Infrared spectroscopy

Q16. Why is acetic acid considered a weak acid?

  • It fully dissociates in water
  • Its conjugate base is unstable
  • Only partial ionization occurs due to resonance-stabilized acetate
  • It reacts slowly with bases

Correct Answer: Only partial ionization occurs due to resonance-stabilized acetate

Q17. What is the primary conjugate base of acetic acid?

  • Acetyl cation
  • Acetate ion
  • Acetaldehyde
  • Acetone

Correct Answer: Acetate ion

Q18. Which equation represents acetic acid dissociation in water?

  • CH3COOH ⇌ CH3COO⁻ + H⁺
  • CH3COOH + H2O → CH3COOH2⁺
  • CH3COOH → CH4 + CO2
  • CH3COOH + OH⁻ → CH3COOCH3

Correct Answer: CH3COOH ⇌ CH3COO⁻ + H⁺

Q19. The acetate ion stabilizes negative charge by which mechanism?

  • Inductive effect from methyl group only
  • Resonance delocalization between two oxygens and the carbonyl
  • Hydrogen bonding with solvent only
  • Charge relocation to hydrogen

Correct Answer: Resonance delocalization between two oxygens and the carbonyl

Q20. Glacial acetic acid has which approximate concentration?

  • ~50% w/w
  • ~70% w/w
  • ~99–100% w/w
  • ~10% w/w

Correct Answer: ~99–100% w/w

Q21. What is the molar mass of acetic acid (approx.)?

  • 46 g/mol
  • 60 g/mol
  • 74 g/mol
  • 88 g/mol

Correct Answer: 60 g/mol

Q22. Which safety hazard is most associated with concentrated acetic acid?

  • Explosive decomposition
  • Severe skin and eye corrosion
  • Radioactivity
  • Spontaneous combustion in air

Correct Answer: Severe skin and eye corrosion

Q23. Which reagent converts acetic acid to acetyl chloride?

  • Thionyl chloride (SOCl2)
  • Sodium hydroxide
  • Hydrogen gas
  • Acetic anhydride

Correct Answer: Thionyl chloride (SOCl2)

Q24. Acetylation using acetic anhydride on salicylic acid produces which drug?

  • Paracetamol
  • Aspirin (acetylsalicylic acid)
  • Ibuprofen
  • Acetaminophen

Correct Answer: Aspirin (acetylsalicylic acid)

Q25. Which buffer system commonly uses acetic acid?

  • Phosphate buffer
  • Tris buffer
  • Acetate buffer
  • Citrate buffer

Correct Answer: Acetate buffer

Q26. During titration of acetic acid with NaOH, the equivalence point pH will be approximately:

  • pH 1
  • pH 7
  • pH 8.7–9
  • pH 12

Correct Answer: pH 8.7–9

Q27. Which statement about acetic acid’s solubility is correct?

  • Insoluble in water
  • Partially miscible with water
  • Completely miscible with water
  • Soluble only in nonpolar solvents

Correct Answer: Completely miscible with water

Q28. The density of glacial acetic acid at 20 °C is approximately:

  • 0.79 g/mL
  • 1.05 g/mL
  • 1.50 g/mL
  • 0.50 g/mL

Correct Answer: 1.05 g/mL

Q29. Which reagent selectively oxidizes ethanol to acetic acid under industrial conditions?

  • Mild acid
  • Catalytic dehydrogenation with oxidants such as air over catalysts
  • Sodium borohydride
  • Hydrochloric acid

Correct Answer: Catalytic dehydrogenation with oxidants such as air over catalysts

Q30. In drug formulation, acetate salts are commonly used to modify which property?

  • Melting point only
  • Solubility and bioavailability
  • Color
  • Optical rotation

Correct Answer: Solubility and bioavailability

Q31. Which of the following indicates the carboxylic proton in 1H NMR of acetic acid?

  • A sharp singlet at ~2 ppm
  • A broad singlet at ~10–12 ppm
  • A triplet at ~1 ppm
  • A doublet at ~7 ppm

Correct Answer: A broad singlet at ~10–12 ppm

Q32. Which is the most appropriate storage recommendation for acetic acid in a pharmacy lab?

  • Store near flammable solvents
  • Store in a cool, well-ventilated area in corrosion-resistant containers
  • Store at high temperatures
  • Store in open containers

Correct Answer: Store in a cool, well-ventilated area in corrosion-resistant containers

Q33. Acetic acid can be used to adjust pH in formulations because it:

  • Is a strong base
  • Provides buffering when combined with acetate
  • Precipitates active ingredients
  • Raises pH above 10

Correct Answer: Provides buffering when combined with acetate

Q34. Which indicator would be suitable for titrating acetic acid with sodium hydroxide?

  • Methyl orange (pH transition ~3.1–4.4)
  • Phenolphthalein (pH transition ~8.2–10)
  • Bromothymol blue (pH transition ~6.0–7.6)
  • Neutral red

Correct Answer: Phenolphthalein (pH transition ~8.2–10)

Q35. Which transformation converts acetic acid to ethanol?

  • Oxidation
  • Reduction
  • Decarboxylation
  • Hydrolysis

Correct Answer: Reduction

Q36. Which of the following is an acetylating agent used in medicinal chemistry?

  • Sodium acetate
  • Acetyl chloride
  • Acetic acid
  • Acetone

Correct Answer: Acetyl chloride

Q37. What happens to acetic acid vapor when inhaled at high concentrations?

  • Acts as an anesthetic
  • Irritates respiratory tract and can cause pulmonary edema
  • Has no effect
  • Reduces blood pressure abruptly

Correct Answer: Irritates respiratory tract and can cause pulmonary edema

Q38. In chromatography, acetic acid is often added to mobile phases to:

  • Increase viscosity only
  • Suppress ionization and improve peak shape for acidic analytes
  • Act as a fluorescent tag
  • Reduce retention of nonpolar compounds only

Correct Answer: Suppress ionization and improve peak shape for acidic analytes

Q39. Which reaction mechanism best describes ester formation from acetic acid and an alcohol under acid catalysis?

  • S_N2 nucleophilic substitution at methyl
  • Nucleophilic acyl substitution (Fischer esterification)
  • Free radical substitution
  • Electrophilic aromatic substitution

Correct Answer: Nucleophilic acyl substitution (Fischer esterification)

Q40. Which statement about acetate buffers is true?

  • They have no buffering capacity near pH 4–5
  • The buffer capacity is maximum near the pKa of acetic acid
  • They are effective only above pH 10
  • They cannot be used in biological systems

Correct Answer: The buffer capacity is maximum near the pKa of acetic acid

Q41. Which reagent will oxidize acetic acid further under normal conditions?

  • PCC (pyridinium chlorochromate)
  • Potassium permanganate under harsh conditions can oxidize derivatives but acetic acid is already fully oxidized carbon
  • Sodium borohydride
  • Hydrogen gas with Pd catalyst

Correct Answer: Potassium permanganate under harsh conditions can oxidize derivatives but acetic acid is already fully oxidized carbon

Q42. Which of the following is NOT a derivative of acetic acid?

  • Acetyl chloride
  • Ethyl acetate
  • Acetone
  • Acetylsalicylic acid

Correct Answer: Acetone

Q43. In mass spectrometry, the molecular ion peak of acetic acid corresponds approximately to which m/z?

  • 18
  • 32
  • 60
  • 88

Correct Answer: 60

Q44. Which interaction explains dimer formation of acetic acid molecules in nonpolar solvents?

  • Ionic bonds between carboxylates
  • Strong hydrogen bonding between carboxyl groups
  • Pi stacking interactions
  • Van der Waals forces only

Correct Answer: Strong hydrogen bonding between carboxyl groups

Q45. Acetic acid’s role in antimicrobial formulations is primarily due to:

  • High molecular weight
  • Low pH and membrane disruption to microbes
  • Oxidative chlorine release
  • Formation of formaldehyde

Correct Answer: Low pH and membrane disruption to microbes

Q46. Which of the following is true about acetic acid’s boiling point?

  • Very low, around 40 °C
  • Moderate, around 118 °C due to hydrogen bonding
  • Extremely high, above 300 °C
  • Same as water

Correct Answer: Moderate, around 118 °C due to hydrogen bonding

Q47. During quality control, titration of a vinegar sample determines acetic acid content by reacting with:

  • Sulfuric acid
  • Sodium hydroxide
  • Silver nitrate
  • Barium chloride

Correct Answer: Sodium hydroxide

Q48. Which type of bond is present between the carbon and the hydroxyl oxygen in the carboxyl group of acetic acid?

  • Single C–O bond and a C=O double bond in the carboxyl group
  • Triple bond
  • Pionic bond
  • Peptide bond

Correct Answer: Single C–O bond and a C=O double bond in the carboxyl group

Q49. Which pharmaceutical process commonly uses acetic acid as a reagent?

  • Hydrogenation to saturate aromatic rings
  • Acetylation of active pharmaceutical ingredients to modify activity or stability
  • Direct photolysis of steroids
  • Formation of azo dyes only

Correct Answer: Acetylation of active pharmaceutical ingredients to modify activity or stability

Q50. Which environmental concern is associated with large-scale acetic acid release?

  • Ozone layer depletion
  • Local acidification and harm to aquatic life
  • Long-term radioactivity
  • Global warming due to greenhouse gas potency

Correct Answer: Local acidification and harm to aquatic life

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