Overview
This lecture explains the concept of the mole in chemistry, Avogadro's number, and various methods to calculate moles using mass, particle count, or gas volume.
Atoms, Molecules, and Size
- Atoms combine to form molecules (e.g., hydrogen + oxygen = water, H₂O).
- Atoms and molecules are extremely small; e.g., tip of a pen contains a trillion hydrogen atoms.
- Diameter of a water molecule is about 0.3 nanometers (10⁻⁹ meters).
The Concept of the Mole
- Counting individual atoms/molecules is impractical due to their tiny size.
- Chemists use "mole" as a counting unit, similar to a dozen (12) or a pair (2).
- One mole equals 6.02 x 10²³ particles (Avogadro's number).
Avogadro’s Hypothesis and Gases
- Avogadro proposed equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
- Regardless of the gas type, a fixed volume contains 6.02 x 10²³ molecules.
Calculating Moles
- Mass method: Number of moles = given mass of substance (g) ÷ molar mass (g/mol).
- Example: 360 g of water ÷ 18 g/mol = 20 moles of water.
- Particle method: Number of moles = number of particles ÷ Avogadro’s constant.
- Example: 3.01 x 10²³ oxygen molecules ÷ 6.02 x 10²³ = 0.5 moles.
- Gas volume method (STP): Number of moles = given volume (L) ÷ 22.4 L.
Key Terms & Definitions
- Atom — Smallest unit of matter that forms chemical elements.
- Molecule — Group of atoms bonded together.
- Mole — Unit representing 6.02 x 10²³ particles of a substance.
- Avogadro’s Constant — 6.02 x 10²³, number of particles in one mole.
- Molar Mass — Mass of one mole of a substance, in grams per mole (g/mol).
- Standard Temperature and Pressure (STP) — 0°C and 1 atm pressure.
Action Items / Next Steps
- Practice calculating moles using mass, particle number, and gas volume.
- Review the periodic table to find molar mass of elements and compounds.