Topics Covered
10 key topics in this chapter
Study Resources
Key Concepts
01. Laws of Chemical Combination
Law of Conservation of Mass (Lavoisier): mass is neither created nor destroyed in a chemical reaction. Law of Constant Proportions (Proust): a compound always contains elements in a fixed mass ratio.
02. Dalton's Atomic Theory
Matter is made of indivisible atoms; atoms of the same element are identical; atoms combine in simple whole-number ratios to form compounds; atoms are neither created nor destroyed in reactions.
03. Atomic Mass & Atomic Mass Unit
Atomic mass is measured relative to carbon-12 (1 amu = 1/12 mass of C-12 atom ≈ 1.66 × 10⁻²⁷ kg). Molecular mass is the sum of atomic masses of all atoms in a molecule.
04. Chemical Formulae
The chemical formula of a compound is written using the valency cross-method: write symbols, exchange valencies (reduce to lowest ratio). E.g., Al³⁺ and O²⁻ → Al₂O₃.
05. The Mole Concept
A mole is the amount of substance containing Avogadro's number (Nₐ = 6.022 × 10²³) of particles. Molar mass = mass of 1 mole in grams = numerically equal to atomic/molecular mass in amu.
06. Mole Calculations
Moles = given mass / molar mass. Number of particles = moles × Nₐ. Volume of gas at STP = moles × 22.4 L. These three relationships are the core of stoichiometry.
Formulas at a Glance
| # | Name | Expression | Notes |
|---|---|---|---|
| 01 | Moles from Mass | n = m / M |
m = given mass (g), M = molar mass (g/mol) |
| 02 | Number of Particles | N = n × Nₐ |
Nₐ = 6.022 × 10²³ mol⁻¹ |
| 03 | Volume at STP | V = n × 22.4 L |
For any gas at STP (0 °C, 1 atm) |
| 04 | Molecular Mass | M = Σ (atomic mass × subscript) |
Sum of all atomic masses in formula |
| 05 | Formula Unit Mass | Same as molecular mass for ionic compounds |
Used for ionic compounds (NaCl, etc.) |
Important Notes
Exam Tips & Common Mistakes
Always show units in mole calculations: n (mol), m (g), M (g/mol), N (particles), V (L).
The cross-valency rule: write cation first, then anion; cross the valencies and reduce to lowest ratio.
If asked to "find the mass of X molecules/atoms", first convert to moles: n = N / Nₐ, then m = n × M.
Molecular mass of H₂SO₄ = 2(1) + 32 + 4(16) = 98 g/mol — a frequently asked value.
State Laws of Conservation of Mass and Constant Proportions clearly in 2-mark questions with examples.