20. Which order correctly arranges the acid strengths of carboxylic acid, water, alcohol and acetylene?
[AIPMT Chemistry — 2003]
(A) RCOOH > ROH > H₂O > HC≡CH
(B) RCOOH > H₂O > ROH > HC≡CH
(C) RCOOH > H₂O > HC≡CH > ROH
(D) RCOOH > HC≡CH > H₂O > ROH
Answer: (B) RCOOH > H₂O > ROH > HC≡CH
Explanation: Typical carboxylic acids have pKa values near 4–5, water about 15.7, simple alcohols about 16–18 and ethyne approximately 25. Lower pKa corresponds to greater acidity. This yields carboxylic acid stronger than water, followed by alcohol and then acetylene.
19. Phospholipids are esters of glycerol with which groups?
[AIPMT Chemistry — 2003]
(A) Three fatty-acid groups
(B) Two fatty-acid groups and a phosphate group
(C) One fatty-acid group and two phosphate groups
(D) Three phosphate groups
Answer: (B) Two fatty-acid groups and a phosphate group
Explanation: A typical glycerophospholipid contains a glycerol backbone esterified to two fatty acid chains and connected to a phosphate-containing head. Its water-loving head and water-repelling tails drive lipid bilayer formation. Such compounds are major structural components of cellular membranes.
18. According to Chargaff’s rule, what equality applies to the bases of double-stranded DNA?
[AIPMT Chemistry — 2003]
(A) A = T and G = C
(B) A = G and T = C
(C) A = C and G = T
(D) All four bases have equal amounts
Answer: (A) A = T and G = C
Explanation: In ordinary double-stranded DNA, adenine pairs with thymine, while guanine pairs with cytosine. This complementary pairing means the number of A bases equals T and the number of G bases equals C. It does not require all four bases to be equally abundant.
17. Which metallic ion is present in the molecular structure of vitamin B12?
[AIPMT Chemistry — 2003]
(A) Iron(II)
(B) Cobalt(III)
(C) Zinc(II)
(D) Calcium(II)
Answer: (B) Cobalt(III)
Explanation: Vitamin B12 is also known as cobalamin because its complex molecular structure contains a cobalt centre coordinated within a corrin ring. Cobalt is essential to this vitamin’s biochemical actions. Iron occurs in haemoproteins instead, while zinc and calcium have unrelated biological functions.
16. What chemical transformation is called glycolysis?
[AIPMT Chemistry — 2003]
(A) Oxidation of glucose to glutamate
(B) Conversion of pyruvate to citrate
(C) Breakdown of glucose into pyruvate
(D) Conversion of glucose into haem
Answer: (C) Breakdown of glucose into pyruvate
Explanation: Glycolysis is a sequence of enzyme-catalysed reactions that breaks one glucose molecule into two pyruvate molecules. Energy from this pathway is conserved as ATP and NADH. It is a central metabolic route and can proceed even without oxygen in many cells.
15. Nylon threads are made from which class of synthetic polymers?
[AIEEE Chemistry — 2003]
(A) Polyesters
(B) Polyamides
(C) Polyethylenes
(D) Polyvinyl compounds
Answer: (B) Polyamides
Explanation: Nylon contains repeating amide bonds in its backbone and therefore belongs to the polyamide family. Strong hydrogen bonding among adjacent chains gives nylon fibre durability and tensile strength. Polyester and polyvinyl materials differ in their fundamental chemical linkages and preparation.
14. What is the first step in sulphuric-acid-catalysed dehydration of an alcohol?
[AIEEE Chemistry — 2003]
(A) Direct carbocation formation
(B) Loss of water before any protonation
(C) Formation of an ester
(D) Protonation of the alcohol oxygen
Answer: (D) Protonation of the alcohol oxygen
Explanation: Strong acid initially protonates the hydroxyl oxygen, converting the poor leaving group OH into a better leaving group, water. Subsequent steps may include loss of water and beta-hydrogen elimination. The mechanism and ease of dehydration depend on whether the alcohol is primary, secondary or tertiary.
13. Why is an ether often more volatile than an isomeric alcohol?
[AIEEE Chemistry — 2003]
(A) Alcohols uniquely have resonance
(B) Ethers form strong intermolecular hydrogen bonds with themselves
(C) Alcohols form intermolecular hydrogen bonds with themselves
(D) Only ethers possess a molecular dipole
Answer: (C) Alcohols form intermolecular hydrogen bonds with themselves
Explanation: Alcohol molecules form hydrogen bonds through their hydroxyl groups, which increase intermolecular attraction and elevate boiling points. Ethers have an oxygen lone pair but no O–H bond, so they cannot donate comparable hydrogen bonds to one another. They therefore often evaporate more readily.
12. What is the correct IUPAC name for CH₃COCH(CH₃)₂?
[AIEEE Chemistry — 2003]
(A) 2-Methylbutan-3-one
(B) 4-Methylisopropyl-1-ketone
(C) 3-Methylbutan-2-one
(D) Isopropylmethyl ketone
Answer: (C) 3-Methylbutan-2-one
Explanation: The longest carbon chain containing the ketone group has four carbon atoms. Numbering from the carbonyl end places the carbonyl carbon at position two and a methyl branch at position three. Hence the systematic name is 3-methylbutan-2-one.
11. Why are the two C–O bonds of the formate ion, HCOO⁻, equal in length?
[AIEEE Chemistry — 2003]
(A) The C=O bond is weaker than a C–O bond
(B) The ion has two equivalent resonance structures
(C) Formate forms when formic acid loses a proton
(D) Carbon has hybridised orbitals
Answer: (B) The ion has two equivalent resonance structures
Explanation: Formate is a resonance hybrid of two equivalent contributing structures, each placing the negative charge on a different oxygen atom. Neither carbon–oxygen bond is exclusively single or double; both therefore have the same intermediate bond order and essentially equal lengths.
