Overview
This lecture covers the basics of stoichiometry, emphasizing core concepts like atomic mass, the mole, molar mass, percent composition, and empirical/molecular formula calculations.
Atomic Mass, Mole, and Molar Mass
- Atomic mass unit (AMU) is 1/12 the mass of a carbon-12 atom (≈ 1.66 × 10⁻²⁷ kg).
- The atomic mass listed on the periodic table is the weighted average of an element's isotopes.
- A mole is a quantity equal to 6.022 × 10²³ entities (Avogadro's number).
- The mass of one mole equals the atomic or molecular mass in grams.
- Molar mass is the mass (in g/mol) of one mole of a substance.
Unit Conversions & Dimensional Analysis
- Divide by Avogadro’s number to convert particles to moles; multiply to go from moles to particles.
- Multiply by molar mass to convert moles to grams; divide by molar mass to convert grams to moles.
- To convert directly from mass to number of atoms: mass → (divide by molar mass) → moles → (multiply by Avogadro’s number) → atoms.
Example Problems
- To find atoms from milligrams of silicon: convert mg to g, divide by molar mass, then multiply by Avogadro’s number.
- To find moles and mass from atoms of cobalt: divide atoms by Avogadro’s number to get moles, multiply by molar mass to get mass.
Percent Composition of Compounds
- Percent composition = (mass of each element in 1 mole of compound) ÷ (molar mass of compound) × 100%.
- Example: For ethanol (C₂H₆O), calculate mass of each element per mole, sum for total mass, then find percent by mass.
Empirical and Molecular Formulas
- Empirical formula shows the lowest whole number mole ratio of elements in a compound.
- Molecular formula is a whole-number multiple of the empirical formula.
- To determine molecular formula: molecular mass ÷ empirical formula mass = multiplier; multiply each subscript of empirical formula by this number.
Determining Empirical Formula from Percent Composition
- Assume 100 g sample: convert percentages to grams for each element.
- Divide each mass by that element’s molar mass to find moles.
- Divide all mole values by the smallest to get ratios; multiply as needed to get whole numbers for empirical formula subscripts.
Key Terms & Definitions
- Atomic Mass Unit (AMU) — 1/12th the mass of a carbon-12 atom.
- Mole — 6.022 × 10²³ entities of a substance.
- Molar Mass — Mass in grams of one mole of a substance.
- Avogadro’s Number — 6.022 × 10²³, the number of particles in a mole.
- Percent Composition — Percentage by mass of each element in a compound.
- Empirical Formula — Smallest whole-number ratio of elements in a compound.
- Molecular Formula — Actual number of atoms of each element in a molecule.
Action Items / Next Steps
- Practice converting between mass, moles, and particle number using dimensional analysis.
- Calculate percent composition and empirical/molecular formulas from sample data.
- Review next lecture topic: chemical equations and balancing.