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If half-life of a… (Class 12 Chemistry SQP 2026-27, Q11)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 1 mark · MCQ

CBSE Class 12 Chemistry sample paper 2026-27: question 11

From the official CBSE sample question paper for 2026-27, section A, question 11, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q11 · 1 mark · MCQChemical Kinetics

If half-life of a first order reaction is 16 minutes, how much time does it take to complete 75% of the reaction?

  1. (A)16 min
  2. (B)32 min
  3. (C)48 min
  4. (D)64 min

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Work out what fraction of the reactant is left after 75% completion.

Hint 2

Count how many half-lives reduce 100% to 25%.

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. For a first order reaction, 75% completion means 25% remains, which is of the initial amount. This takes two half-lives.

  2. Time min.

    1 mark

Answer: 32 min, option (B)

Where the marks are

  • Recognises 75% completion = 2 half-lives, giving 32 min (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Chemical Kinetics, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: 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.