Overview
This lecture introduces the mole as a counting unit in chemistry and explains how it simplifies the measurement and conversion of atoms, molecules, and ions in chemical reactions.
The Concept of the Mole
- Atoms combine in fixed ratios to form molecules (e.g., water is always 2 hydrogen:1 oxygen).
- Counting individual atoms is impractical due to their tiny size and large quantities.
- The mole (mol) is a unit for counting large numbers of atoms or molecules.
- One mole equals 6.022 × 10²³ entities, known as Avogadro's number.
The Mole and Chemical Entities
- The mole can refer to atoms, molecules, formula units, or ions—specify which one.
- One mole of an element consists of Avogadro’s number of atoms (e.g., 1 mol He = 6.022 × 10²³ He atoms).
- One mole of a molecular element or compound contains Avogadro’s number of molecules (e.g., 1 mol N₂ = 6.022 × 10²³ N₂ molecules).
- For ionic compounds, one mole equals Avogadro’s number of formula units (e.g., 1 mol NaCl = 6.022 × 10²³ NaCl units).
Mass Relationships and the Mole
- The mass ratio between atoms remains constant for any number of atoms (e.g., oxygen to hydrogen is always 16:1).
- Samples with equal moles of different elements contain equal numbers of atoms, regardless of mass.
- Chemists use moles to relate macroscopic samples to the number of particles.
Converting Between Moles, Atoms, Molecules, and Ions
- Use chemical formulas to relate moles of compounds to moles of constituent atoms.
- Example: 1 mol C₂H₆O contains 2 mol C, 6 mol H, and 1 mol O atoms.
- To convert: multiply the given moles by the subscript number from the formula.
- Avogadro’s number serves as a conversion factor between moles and number of particles:
- 1 mol X = 6.022 × 10²³ X atoms/molecules/units.
Calculation Examples
- 2.5 mol C₂H₆O contains 5 mol C, 15 mol H, and 2.5 mol O atoms.
- 2.34 mol NaCl equals 1.41 × 10²⁴ NaCl units; since each unit has 2 ions, that's 2.82 × 10²⁴ ions.
Key Terms & Definitions
- Mole (mol) — unit representing 6.022 × 10²³ items (Avogadro’s number).
- Avogadro’s Number — 6.022 × 10²³, the number of entities in one mole.
- Formula Unit — smallest ratio of ions in an ionic compound.
- Chemical Entity — general term for atoms, molecules, or ions being counted.
Action Items / Next Steps
- Practice converting between moles and number of atoms/molecules using chemical formulas.
- Complete exercises at the end of the section to reinforce conversion techniques.
- Review the next section: Atomic and Molar Masses.