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Atoms and Molecules — Question 2

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Question 2

When 3.0 g of carbon is burnt in 8.00 g of oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

Answer

Given,

C 3 g+O2 8 g⟶CO211 g\underset{\text{ 3 g}}{\text{C}} + \underset{\text{ 8 g}}{\text{O}_2} \longrightarrow \underset{\text{11 g}}{\text{CO}_2}

When 3.0 g of carbon is burnt in 8.00 g of oxygen, 11.00 g of carbon dioxide is produced.

Now,

C 3 g+O2 50 g⟶CO2?\underset{\text{ 3 g}}{\text{C}} + \underset{\text{ 50 g}}{\text{O}_2} \longrightarrow \underset{\text{?}}{\text{CO}_2}

Carbon dioxide contains carbon and oxygen in a fixed ratio by mass, which is 3:8. Hence, 3 g of carbon will react with 50 g of oxygen but only 8 g of it will be used in producing 11 g of CO2 and 42 g [i.e., 50 g - 8 g] of oxygen will be left unused.

The above answer is governed by the law of constant proportions.

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Science | Chapter 3: Atoms and MoleculesWeb Content — Quick Reference

Chapter 3: Atoms and Molecules — Quick Reference

Dalton theory atomic mass molecular mass mole concept chemical formula valency Avogadro number

Quick Revision Points

  • Law of conservation of mass: total mass unchanged in a reaction
  • Law of constant proportions: elements in fixed mass ratio in a compound
  • 1 u = 1/12 mass of C-12 = 1.66 × 10⁻²⁴ g
  • Molecular mass = sum of atomic masses of all atoms in molecule
  • 1 mole = 6.022 × 10²³ particles (Avogadro's number)
  • Moles = mass / molar mass; Particles = moles × Nₐ
  • Criss-cross method for writing chemical formulae using valencies
  • Polyatomic ions: NH₄⁺, OH⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, PO₄³⁻
Exam Tips for Chapter 3
  • Read the detailed chapter notes for complete coverage of all NCERT topics.
  • Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
  • For numericals (if applicable), practice at least 20 problems of varying difficulty.
  • Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
  • Detailed Notes: ch03-atoms-and-molecules.html
  • Practice Questions: 100+ questions with answers in 05-practice-questions/
  • Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
  • Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
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