Overview
This lecture introduces the mole concept in chemistry, explains its calculations, and covers related terms such as atomic mass, molecular mass, and molar mass with sample problems.
The Mole Concept
- Counting atoms or molecules individually is impractical due to their tiny size.
- Scientists use the mole as a counting unit, similar to "dozen" or "ream."
- One mole is defined as the number of particles in exactly 12 grams of carbon-12.
- Avogadro's constant (Nₐ) is 6.02 × 10²³ particles per mole.
- A mole can represent atoms, molecules, ions, or other particles.
Calculating Moles and Particles
- The number of moles (n) = total number of particles (N) ÷ Avogadro's constant (Nₐ).
- One mole of a molecule may contain multiple moles of its constituent elements (e.g., 1 mole H₂O contains 2 moles H and 1 mole O).
- Conversion steps: Identify given/unknown, choose correct conversion factor, and perform calculation.
Example Problems
- 4 moles NaCl × 6.02 × 10²³ molecules/mole = 2.41 × 10²⁴ molecules NaCl.
- 3.01 × 10²² atoms Mg ÷ 6.02 × 10²³ atoms/mole = 5 × 10⁻² moles Mg.
- 1.46 moles AlF₃ × 6.02 × 10²³ units/mole × 3 F⁻/unit = 2.64 × 10²⁴ fluoride ions.
Atomic, Molecular, and Molar Mass
- Atomic mass: mass of one atom, measured in atomic mass units (amu).
- Molecular mass: sum of atomic masses of all atoms in a molecule, in amu.
- Example: H₂O molecular mass = 2×1.00 + 15.9994 = 18.0154 amu.
- Molar mass: mass of one mole of a substance, usually in grams/mole.
- Molar mass formula: mass (g) ÷ number of moles.
- Example: NaCl molar mass = 58.44 g/mole.
Key Terms & Definitions
- Mole — unit for amount of substance; 6.02 × 10²³ particles.
- Avogadro's constant (Nₐ) — 6.02 × 10²³ particles/mole.
- Atomic mass — mass of a single atom in amu.
- Molecular mass — sum of atomic masses in a molecule, in amu.
- Molar mass — mass of one mole of substance, in g/mole.
Action Items / Next Steps
- Prepare for the next lesson on percentage composition of a compound.