Mole Concept

• Mole (mol) — amount of substance containing 6.022 × 10²³ entities (Avogadro number N_A).
• Molar mass — mass of 1 mole in grams; numerically equal to atomic/molecular mass in u.
• n = mass / molar mass; particles = n × N_A.
• Links microscopic atoms to macroscopic grams.

1. Avogadro number — N_A = 6.022 × 10²³ mol⁻¹.
2. Molar mass of H₂O — 18 g/mol (18 u → 18 g for one mole).
3. Number of moles n = given mass (g) / molar mass (g/mol).
4. Number of particles = n × 6.022 × 10²³.
5. Diagram (text) — 18 g water ≈ one mole ≈ 6 × 10²³ molecules.
6. NCERT Ch. 3 — convert grams to moles and to particle count.
7. Real world — chemists use moles for reaction stoichiometry.
8. Diatomic elements — O₂ molar mass = 32 g/mol, not 16.

How many moles in 88 g of CO₂? (Molar mass = 44 g/mol)

Step 1 — n = mass / molar mass = 88 / 44 = 2 mol
Step 2 — Number of molecules = 2 × 6.022 × 10²³ = 1.204 × 10²⁴
Step 3 — Number of O atoms = 2 × 2 × 6.022 × 10²³ (2 O per CO₂)
Step 4 — = 4 × 6.022 × 10²³ ≈ 2.41 × 10²⁴ atoms
Conclusion: always identify what entity (molecule vs atom) is counted.

• Using atomic mass instead of molar mass for diatomic gases (O₂ = 32 g/mol).
• Misconception: mole is a mass unit (it's amount of substance).
• Confusing number of moles with number of molecules.
• Forgetting coefficients when counting atoms in compounds.
• Rounding N_A too early in multi-step problems.

• Define mole and Avogadro number.
• Calculate moles in 9 g of water.
• How many molecules in 0.5 mol of O₂?
• What is the mass of 3 mol of NaCl? (M = 58.5 g/mol)

Open the Practice tab for graded questions on Mole Concept.