PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2025-26: question 24

From the official CBSE sample question paper for 2025-26, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerNuclei

A fast-moving neutron collides with the nucleus of Plutonium (Pu), thereby producing Xenon (Xe) and Zirconium (Zr) along with neutrons.
(I) Write the nuclear fission reaction.
(II) Find the energy released in the above nuclear reaction.
Given atomic masses:
m( $_{94}^{239} P u$ ) = 239.052157u,
m( $_{40}^{103} Z r$ ) = 102.926597u,
m( $_{54}^{134} X e$ ) = 133.905040u &
m( $_{0}^{1} n$ ) = 1.00866u.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Balance mass numbers and atomic numbers to find how many neutrons are released.

Hint 2

Mass defect = total initial mass − total final mass.

Hint 3

1 u is equivalent to 931.5 MeV.

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.

(I) $_{94}^{239} Pu +_{0}^{1} n \to_{54}^{134} Xe +_{40}^{103} Zr + 3_{0}^{1} n$ (mass number $240 = 134 + 103 + 3$ ; charge $94 = 54 + 40$ ).
1 mark
(II) Mass defect: $Δ m = [m (Pu) + m (n)] - [m (Xe) + m (Zr) + 3 m (n)] = [239.052157 + 1.00866] - [133.905040 + 102.926597 + 3.02598]$
$Δ m = 240.060817 - 239.857617 = 0.2032 u$
1 mark
Energy released $Q = Δ m \times 931.5 MeV/u = 0.2032 \times 931.5 \approx 189.3 MeV$
1 mark

Answer: (I) Pu-239 + n → Xe-134 + Zr-103 + 3n; (II) Δm = 0.2032 u, Q ≈ 189.3 MeV

Where the marks are

Correct balanced fission equation with 3 neutrons (1 mark)
Mass defect 0.2032 u (1 mark)
Energy released ≈ 189.3 MeV (1 mark)

Revise the chapter

Chapter: Nuclei (notes, formulas, where marks are lost and every past question)
Topic: Nuclear Fission and Fusion

Practise this chapterBoard-style questions from Nuclei, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

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CBSE Class 12 Physics sample paper 2025-26: question 24

Hints

Step-by-step solution

Revise the chapter

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