Overview
This lecture introduces Chapter 6: Chemical Composition, focusing on the concepts of the mole, molar mass, and how these relate to calculations involving atoms, molecules, and compounds.
Main Concepts in Chemical Composition
- Chapter 6 covers mole concept, percentage (mass) composition, empirical formula, and molecular formula.
- The mole links atomic-scale measurements to macroscopic amounts in chemistry.
- Avogadro’s number (6.022 × 10²³) defines one mole, similar to a “dozen” meaning twelve.
- One mole of any element contains Avogadro’s number of atoms; one mole of a compound contains Avogadro’s number of molecules.
The Mole and Molar Mass
- Molar mass is the mass of one mole of a substance, found using the periodic table for elements.
- Molar mass for compounds is the sum of atomic masses of all atoms in the formula.
- Masses of moles for different elements are different, even though the number of particles is the same.
Conversion Pathways Between Mass, Mole, Atoms, Molecules
- Four key formulas connect mass, mole, and number of particles:
- Mole → Mass: given moles × molar mass
- Mass → Mole: given mass ÷ molar mass
- Mole → Atoms/Molecules: given moles × Avogadro’s number
- Atoms/Molecules → Mole: given particles ÷ Avogadro’s number
- For compounds, use “molecules” instead of “atoms” in calculations.
Example Problem Approaches
- Identify what’s given (mass, mole, or number of particles) and what’s being asked.
- Use the appropriate formula(s) based on starting and ending units.
- For two-step problems (e.g., mass → moles → molecules), solve sequentially.
- Always use correct units and check for scientific notation.
Problem Examples Covered
- Convert moles of element to mass.
- Calculate number of atoms from given moles.
- Find mass from number of atoms or molecules using two-step conversion.
- Calculate moles from mass for compounds using calculated molar mass.
- Find number of molecules from given mass for compounds.
Key Terms & Definitions
- Mole (mol) — Unit representing 6.022 × 10²³ particles (atoms/molecules).
- Avogadro’s Number — 6.022 × 10²³; the number of particles in one mole.
- Molar Mass — Mass of one mole (in grams) of a substance.
- Formula Mass — Total atomic masses in a compound’s formula.
- Atoms — Building blocks of elements.
- Molecules — Building blocks of compounds.
Action Items / Next Steps
- Practice using the four mole conversion formulas.
- Read textbook sections 6.1–6.5 for mole concept and conversions.
- Prepare for next class: focus will be on percent composition.