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Atoms and Molecules — Question 6

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Question 6

Calculate the molar mass of the following substances.

(a) Ethyne, C2H2

(b) Sulphur molecule, S8

(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO3

Answer

(a) Molar mass of Ethyne C2H2 = (2 x Mass of C) + (2 x Mass of H) = (2 × 12) + (2 × 1) = 24 + 2 = 26 g

(b) Molar mass of Sulphur molecule S8 = (8 x Mass of S) = 8 x 32 = 256 g

(c) Molar mass of Phosphorus molecule, P4 = 4 x Mass of P = 4 x 31 = 124 g

(d) Molar mass of Hydrochloric acid, HCl = Mass of H + Mass of Cl = 1 + 35.5 = 36.5 g

(e) Molar mass of Nitric acid, HNO3 = Mass of H + Mass of N + (3 x Mass of O) = 1 + 14 + (3 × 16) = 15 + 48 = 63 g

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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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