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Give reasons for any 3 of… (Class 12 Chemistry 2023, Q29)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2023: question 29

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section C, question 29, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 3 marks · Short answerBiomolecules

Give reasons for any 3 of the following observations :
(a) Penta-acetate of glucose does not react with hydroxylamine.
(b) Amino acids behave like salts.
(c) Water soluble vitamins must be taken regularly in diet.
(d) The two strands in DNA are complimentary to each other.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

For (a), ask whether the ring of pentaacetate can open to give an aldehyde.

Hint 2

For (b), consider what happens when an acid group and a base group are in the same molecule.

Hint 3

For (c) and (d), think about urinary excretion and specific base pairing respectively.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) In glucose pentaacetate the –OH at C-1 is acetylated, so the ring cannot open to give a free –CHO group. Without a free aldehyde group, it cannot form an oxime with hydroxylamine.

    1 mark

  2. (b) Amino acids contain both an acidic (–COOH) and a basic (–NH₂) group, which react internally to form a dipolar zwitter ion . Being ionic, they show salt-like properties (high m.p., water solubility).

    1 mark

  3. (c) Water-soluble vitamins (B-complex, C) are readily excreted in urine and cannot be stored in the body (except B₁₂), so they must be supplied regularly through diet.

    1 mark

  4. (d) The bases pair specifically through hydrogen bonds — adenine with thymine (2 H-bonds) and guanine with cytosine (3 H-bonds). Hence the base sequence of one strand fixes the sequence of the other, making them complementary. (Any three of (a)–(d) earn full 3 marks.)

Answer: (a) no free –CHO; (b) zwitter ion formation; (c) excreted in urine, not stored; (d) specific base pairing A–T, G–C

Where the marks are

  • Absence of free –CHO group in pentaacetate (1 mark)
  • Zwitter ion due to –COOH and –NH₂ in same molecule (1 mark)
  • Water-soluble vitamins excreted in urine/not stored (1 mark)
  • Specific H-bonded base pairing A–T and G–C (1 mark; any three parts for 3 marks)

Revise the chapter

Practise this chapterBoard-style questions from Biomolecules, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 56/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.