Understanding Formula Mass and Moles

Sep 4, 2024

Lecture Notes: Formula Mass, The Mole, and Avogadro's Number

Key Topics

  • Formula Mass
  • Mole and Avogadro's Number
  • Relation Between Mass, Moles, and Number of Atoms/Molecules
  • Calculations and Conversions

Formula Mass

  • Definition: Sum of average atomic masses of all atoms in a formula.
  • Covalent Compounds: Known as molecular mass.
    • Example: Chloroform (CHCl₃)
      • Calculation: Sum of atomic masses of C, H, and Cl multiplied by their number in the molecule.
  • Ionic Compounds: Known as formula mass.
    • Ionic substances consist of discrete cations and anions.
    • Example: Sodium chloride (NaCl)
      • Calculation: Sum of atomic masses of Na and Cl.

Mole and Avogadro's Number

  • Mole: A unit for amount of substance.
    • Analogous to terms like pair (2), dozen (12), gross (144).
    • Definition: Amount of substance with as many entities as in 12 grams of pure carbon-12.
  • Avogadro's Number:
    • Value: Approximately 6.022 x 10²³.
    • Represents the number of atoms or molecules in one mole.
    • Named after Amedeo Avogadro.

Molar Mass

  • Definition: Mass of one mole of a substance, expressed in grams per mole (g/mol).
  • Relationship: Numerically equivalent to atomic or formula mass in AMU.
    • Conversion between AMU and g/mol due to definition of mole.

Calculations and Conversions

Mass to Moles

  • Example: 4.7 grams of potassium.
    • Calculation: Divide mass by molar mass (from periodic table) to convert to moles.

Moles to Mass

  • Example: 9.2 x 10⁴ moles of argon.
    • Calculation: Multiply moles by molar mass to find mass.

Mass to Atoms

  • Example: 24 grams of copper.
    • Calculation: Convert mass to moles, then multiply by Avogadro’s number to find atoms.

Molecule Calculations

  • Example: Glycine (C₂H₅NO₂)
    • Determine molar mass by calculating total atomic masses.
    • Calculate moles from given mass.

Complex Calculations

  • Example: Saccharin molecule count from mass.
    • Given molar mass can simplify calculations.
    • Convert mass to moles, then use formula to determine number of specific atoms (e.g., carbon atoms).

Tips for Calculations

  • Diagramming conversions can clarify steps.
  • Use parentheses in calculations to ensure correct order of operations.
  • Moles serve as intermediary for converting between mass and number of atoms.

These notes summarize the key concepts and processes involved in calculating formula masses, understanding the mole, and performing conversions between mass, moles, and atoms. Remember to refer to specific examples discussed in class to reinforce these concepts.