Overview
This lecture covers molar mass, mole calculations, Avogadro’s constant, calculating numbers of particles, and determining masses in chemical reactions using stoichiometry.
Molar Mass and Mole Calculations
- Molar mass (M) is the mass of one mole of a substance, measured in g/mol.
- For elements, molar mass equals the element's relative atomic mass.
- For compounds, molar mass equals the compound's relative molecular or formula mass.
- The formula for moles: amount of substance (mol) = mass (g) ÷ molar mass (g/mol).
Example Calculations with Moles
- To find mass: multiply the number of moles by molar mass (e.g., 0.75 mol S × 32 g/mol = 24 g S).
- To find moles: divide the mass by molar mass (e.g., 10.2 g CaCO₃ ÷ 100 g/mol = 0.102 mol).
- For compounds, sum the atomic masses of all atoms to get the molar mass.
Avogadro’s Constant and Particle Calculations
- One mole of any substance contains 6.02 × 10²³ particles (Avogadro's constant).
- Number of molecules = moles × Avogadro’s number (e.g., 0.625 mol CH₄ × 6.02 × 10²³ = 3.76 × 10²³ molecules).
- Number of atoms in a molecular sample = molecules × number of atoms per molecule.
Calculating Reacting Masses (Stoichiometry)
- Use balanced equations to determine mole ratios between reactants and products.
- Example: 6 g Ca reacts to produce 8.4 g CaO; first calculate moles, use mole ratio, then mass.
- Example: 250 kg Fe₂O₃ converts to 175 kg Fe; convert mass to grams, calculate moles, apply mole ratio, and convert final mass as needed.
Key Terms & Definitions
- Molar Mass (M) — Mass of 1 mole of a substance, in g/mol.
- Mole (mol) — The unit for amount of substance, representing 6.02 × 10²³ particles.
- Relative Atomic Mass — Average mass of an atom, compared to 1/12 of a carbon-12 atom.
- Relative Molecular Mass — Sum of relative atomic masses in a molecule.
- Avogadro’s Constant — Number of particles in one mole (6.02 × 10²³).
Action Items / Next Steps
- Practice mole and mass calculations for both elements and compounds.
- Complete textbook exercises on calculating reacting masses with balanced equations.