ScienceCBSE Class 10 · Sample paper, session 2024-25 · 1 mark · MCQ

CBSE Class 10 Science sample paper 2024-25: question 1

From the official CBSE sample question paper for 2024-25, section A, question 1, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q1 · 1 mark · MCQChemical Reactions and Equations

Identify 'p', 'q' and 'r' in the following balanced reaction

$p Pb (N O_{3})_{2 (s)} H e a t q Pb O_{(s)} + r N O_{2 (g)} + O_{2 (g)}$

(A)2,2,4
(B)2,4,2
(C)2,4,4
(D)4,2,2

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Count the N and O atoms on each side.

Hint 2

Each Pb(NO₃)₂ gives two NO₂. Try 2 molecules of lead nitrate with one O₂.

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.

Write the skeletal equation: $Pb (N O_{3})_{2} Heat PbO + N O_{2} + O_{2}$ . Balance N and O first, keeping the coefficient of $O_{2}$ as 1.
Take $p = 2$ . Then Pb gives $q = 2$ and N gives $2 \times 2 = 4$ , so $r = 4$ : $2Pb (N O_{3})_{2} \to 2PbO + 4N O_{2} + O_{2}$ .
½ mark
Check oxygen. Left side: $2 \times 6 = 12$ . Right side: $2 + 8 + 2 = 12$ . The equation is balanced, so $p, q, r = 2, 2, 4$ , which is option (A).
½ mark

Answer: p = 2, q = 2, r = 4; option (A)

Where the marks are

Balances Pb and N atoms correctly (0.5 mark)
Verifies oxygen balance and selects (A) 2,2,4 (0.5 mark)

Revise the chapter

Chapter: Chemical Reactions and Equations (notes, formulas, where marks are lost and every past question)
Topic: Balancing Chemical Equations

Practise this chapterBoard-style questions from Chemical Reactions and Equations, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

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CBSE Class 10 Science sample paper 2024-25: question 1

Hints

Step-by-step solution

Revise the chapter

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