Science · Class 9
Atoms and Molecules
Laws of chemical combination, mole concept, and molecular mass.
3 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
One mole contains 6.022 × 10²³ particles (Avogadro number).
Laws of chemical combination lead to atomic theory. You study Dalton's postulates, symbols, atomic mass, molecules, ions, writing chemical formulae, and mole concept — foundation for stoichiometry.
Laws of Chemical Combination
- Law of conservation of mass: mass neither created nor destroyed
- Law of constant proportions: elements in fixed ratio by mass
Mole Concept
Moles
n = mass (g) / molar mass (g/mol)
Particles
Number of particles = n × N_A ; N_A = 6.022 × 10²³ mol⁻¹
Valency rule
Cross-multiply valencies to write formula (e.g. Ca²⁺, Cl⁻ → CaCl₂)
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Moles from mass
How many moles in 88 g CO₂? (M = 44 g/mol)
- 1n = 88/44 = 2 mol
Answer
2 mol
Formula writing
Write formula of aluminium oxide.
- 1Al³⁺ and O²⁻
- 2Cross valencies: Al₂O₃
Answer
Al₂O₃
Key Points to Remember
- ✓ Atomicity: number of atoms in one molecule (O₂ = 2)
- ✓ Polyatomic ions: SO₄²⁻, NO₃⁻, NH₄⁺
- ✓ Molecular mass = sum of atomic masses
Exam Tips
- • Balance valencies when writing formulae
- • Use mole concept for particle count problems
- • Memorise common ion formulae
Practice MCQs — Atoms and Molecules
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Avogadro number is approximately: