Overview
This lecture explains the concept of the mole in chemistry, its relation to Avogadro's number, molar mass, and demonstrates how to convert between grams, moles, and number of particles.
The Mole Concept
- A mole is a unit that groups 6.022 × 10²³ atoms, molecules, or other particles.
- Similar to terms like dozen (12 items) or ream (500 sheets), but used for microscopic particles.
- Avogadro’s number (6.022 × 10²³) defines the number of particles in one mole.
- Use "atoms" for moles of a single element, and "molecules" for compounds (multiple elements).
Molar Mass and Calculations
- The molar mass of an atom in grams equals its atomic mass unit (amu) value from the periodic table.
- Example: 1 mole of hydrogen = 1 gram, 1 mole of magnesium = 24 grams.
- For molecules: add the atomic masses of each atom in the molecule to get molecular mass in grams (molar mass).
- Example: 1 mole of H₂O = 2(1) + 16 = 18 grams.
- Example: 1 mole of H₂SO₄ = 2(1) + 32 + 4(16) = 98 grams.
Converting Between Moles, Grams, and Particles
- To convert moles to grams: multiply the number of moles by the molar mass.
- Example: 0.57 mol Al × 27 g/mol = 15.39 g Al
- To convert grams to moles: divide grams by the molar mass.
- Example: 4.3 g Mn ÷ 55 g/mol = 0.078 mol Mn
- To convert moles to particles (atoms/molecules): multiply moles by Avogadro’s number.
- Example: 4.8 mol Ca × 6.022 × 10²³ = 2.9 × 10²⁴ atoms Ca
- To convert particles to moles: divide by Avogadro’s number.
- Example: 2.35 × 10²⁴ atoms Cu ÷ 6.022 × 10²³ = 3.9 mol Cu
Multi-Step Conversions
- To convert grams to atoms: grams → moles (divide by molar mass) → atoms (multiply by Avogadro's number).
- Example: 24 g Mg ÷ 24.3 g/mol × 6.022 × 10²³ = 5.95 × 10²³ atoms Mg
- For atoms to grams: reverse the steps (atoms → moles → grams).
Key Terms & Definitions
- Mole (mol) — Unit representing 6.022 × 10²³ particles.
- Avogadro’s Number — 6.022 × 10²³, the number of particles in a mole.
- Molar Mass — Mass (in grams) of 1 mole of a substance; equal to the atomic/molecular mass in amu, but expressed in grams.
- Molar Factor — The ratio used to convert between moles and grams (e.g., 1 mol H₂SO₄ = 98 g H₂SO₄).
Action Items / Next Steps
- Practice converting between grams, moles, and particles using different substances.
- Review how to calculate molar mass from atomic masses.
- Prepare for quizzes or homework by solving example problems on mole conversions.