Overview
This lecture explains the concept of the mole in chemistry, Avogadro's number, and how to perform conversions between moles, grams, atoms, molecules, and formula units.
The Concept of the Mole
- A mole is a counting unit like a dozen, but represents 6 × 10²³ items (Avogadro's number).
- The mole is used to count very small particles (atoms, molecules, or formula units).
Avogadro's Number and Its Use
- Avogadro's number, 6 × 10²³, is the number of particles in one mole of a substance.
- Used for conversion between moles and number of particles (atoms, molecules, or formula units).
Converting Between Moles and Particles
- To convert moles to atoms: multiply moles by Avogadro's number.
- Example: 4 moles of C × 6 × 10²³ = 2.4 × 10²⁴ atoms of carbon.
- Keep scientific notation between 1 (inclusive) and 10 (exclusive).
Atoms, Molecules, and Formula Units
- Atoms: single elements (e.g., C, Zn, Ne).
- Molecules: two or more nonmetal atoms bonded together (e.g., H₂, H₂O, CH₄).
- Formula units: ionic compounds of metals and nonmetals (e.g., NaCl, AlCl₃).
Conversion Example Practice
- 5 moles of CH₄ = 3.0 × 10²⁴ molecules of CH₄.
- Each CH₄ molecule has 4 H atoms, so 1.2 × 10²⁵ H atoms in 5 moles CH₄.
- 4 moles of AlCl₃ = 2.4 × 10²⁴ formula units of AlCl₃.
- Each AlCl₃ has 3 Cl⁻ ions, totaling 7.2 × 10²⁴ Cl⁻ ions.
Working Backwards: Particles to Moles
- To convert particles to moles: divide the number of particles by Avogadro's number.
Molar Mass Calculations
- Molar mass = sum of atomic masses in a compound (in grams/mole).
- Example: C₂H₆ → (2×12) + (6×1) = 30 g/mol.
- Example: Glucose (C₆H₁₂O₆) → 180 g/mol.
Grams to Moles Conversions
- To convert grams to moles: divide mass by molar mass.
- Example: 34 g NH₃ ÷ 17 g/mol = 2 moles NH₃.
Moles to Grams Conversions
- To convert moles to grams: multiply moles by molar mass.
- Example: 3 moles Ne × 20 g/mol = 60 g Ne.
Grams to Atoms and Atoms to Grams
- Grams → Moles → Atoms: convert grams to moles, then moles to atoms using Avogadro's number.
- Atoms → Moles → Grams: convert atoms to moles, then moles to grams using molar mass.
Key Terms & Definitions
- Mole — unit representing 6 × 10²³ particles.
- Avogadro's Number — 6 × 10²³; the number of particles in a mole.
- Molar Mass — mass of one mole of a substance, in g/mol.
- Formula Unit — smallest repeating unit of an ionic compound.
Action Items / Next Steps
- Practice converting between moles, grams, atoms, molecules, and formula units.
- Calculate molar masses for given compounds using the periodic table.