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Chemical Equations and Moles

Jul 14, 2025

Overview

This lecture covers how to use chemical equations and the concept of the mole to convert between mass, moles, and the number of atoms/molecules, and how these conversions apply to chemical reactions (stoichiometry).

The Mole and Molar Mass

  • A mole is a named number: 1 mole = 6.022 x 10²³ (Avogadro's number).
  • The mass of 1 mole of an element in grams equals its atomic mass from the periodic table.
  • Molar mass is the mass (in grams) of 1 mole of a substance.
  • To calculate molar mass of a compound, sum the masses of all atoms in its formula.

Converting Between Grams and Moles

  • Use molar mass as a conversion factor: grams ↔ moles.
  • Example: To find grams in 4.2 moles of Na₂SO₄, multiply moles by molar mass.
  • Example: To find moles in a given mass, divide mass by molar mass.

The Line Method (Dimensional Analysis)

  • Set up conversion "lines" to string together ratios and cancel units.
  • Always align units so they cancel out step by step toward the desired unit.

Stoichiometry and Chemical Equations

  • Balanced chemical equations provide mole ratios between reactants and products.
  • Use coefficients to convert moles of one substance to moles of another.
  • Example: For 4Fe + 3O₂ → 2Fe₂O₃, 4 moles Fe reacts with 3 moles O₂ to form 2 moles Fe₂O₃.
  • Always use coefficients from the chemical equation when changing substances.

Multi-step Conversions

  • For grams A → grams B:
    1. Convert grams A to moles A (using molar mass).
    2. Convert moles A to moles B (using reaction coefficients).
    3. Convert moles B to grams B (using molar mass).
  • Always relate one mole of substance to its molar mass except for stoichiometric ratios.

Key Terms & Definitions

  • Mole — A quantity of 6.022 x 10²³ particles (atoms/molecules/formula units).
  • Avogadro's Number — 6.022 x 10²³, the number of particles in a mole.
  • Molar Mass — Mass of one mole of a substance, typically in g/mol.
  • Stoichiometry — Calculating quantities in chemical reactions using balanced equations.
  • Coefficient — The number in front of compounds/elements in a balanced equation, indicates mole ratio.

Action Items / Next Steps

  • Practice setting up and solving gram↔mole and stoichiometry problems.
  • Review calculation of molar mass for elements and compounds using the periodic table.
  • Complete assigned homework problems on stoichiometry and conversions.
  • Bring questions about confusing steps to the next class or office hours.

Full transcript