Understanding Moles and Chemical Formulas

⚖️ 1. Mole Concept

Just like “a dozen” means 12 items, 1 mole = 6.022 × 1023 particles (atoms, molecules, etc.). This number is called Avogadro’s Number (NA).

Examples:

  • 1 mole H atoms = 6.022 × 1023 H atoms
  • 1 mole H2O = 6.022 × 1023 water molecules

📊 2. Molar Mass

Molar mass = mass of 1 mole of a substance in grams (numerically equal to atomic/molecular mass in “u”).

Examples:

  • Molar mass of H2O = 18.02 g/mol
  • Molar mass of NaCl = 58.5 g/mol

🧮 3. Percentage Composition

Formula to find mass % of an element:

\[ \text{Mass \%} = \frac{\text{Mass of element in compound}}{\text{Molar mass of compound}} \times 100 \]

Example for H2O:

  • H % = (2×1.008 / 18.02) × 100 = 11.18%
  • O % = (16.00 / 18.02) × 100 = 88.79%

🔍 4. Empirical & Molecular Formulas

Empirical formula: Simplest whole-number ratio of atoms.
Molecular formula: Actual atom counts in a molecule.

Steps to find formulas (Example: 4.07% H, 24.27% C, 71.65% Cl, Molar mass=98.96 g/mol):

  1. Assume 100g sample → H:4.07g, C:24.27g, Cl:71.65g
  2. Convert to moles:
    • H: 4.07 / 1.008 = 4.04 mol
    • C: 24.27 / 12.01 = 2.021 mol
    • Cl: 71.65 / 35.453 = 2.021 mol
  3. Divide by smallest mole value (2.021):
    H:C:Cl = 4.04/2.021 : 2.021/2.021 : 2.021/2.021 = 2:1:1
    → Empirical formula = CH2Cl
  4. Find molecular formula:
    • Empirical mass = 12.01 + 2×1.008 + 35.453 = 49.48 g/mol
    • n = Molar mass / Empirical mass = 98.96 / 49.48 ≈ 2
    • Molecular formula = (CH2Cl)×2 = C2H4Cl2

⚗️ 5. Stoichiometry

Uses balanced equations to calculate reactant/product masses/volumes.
Example (methane combustion):

\[ \ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)} \]

Key relationships:

  • 1 mole CH4 requires 2 moles O2
  • 16g CH4 produces 44g CO2
  • 22.7L CH4 produces 22.7L CO2

Use: mass ⇔ moles ⇔ molecules ⇔ volume

🚦 6. Limiting Reagent

The reactant that gets used up first and limits the product amount in a reaction.

🌟 NEET Super-Important Concepts 🌟

  1. 💡 Mole calculations (using Avogadro’s number: 6.022 × 1023)
  2. 🧪 Percentage composition and empirical/molecular formulas
  3. ⚖️ Stoichiometric calculations from balanced equations
  4. ⚠️ Identifying the limiting reagent
  5. 📐 Molar mass determinations for compounds