Lecture Notes on Chemistry Calculations

Isotope Average Atomic Mass Calculation

• Example: Calculate average atomic mass of Bromine.
• Isotopes:
  • Bromine-79: 78.9183 amu, 50.69% abundance
  • Bromine-81: 80.9163 amu, 49.31% abundance
• Formula:
  • Average mass = (mass of Isotope 1 * % abundance as decimal) + (mass of Isotope 2 * % abundance as decimal)
• Calculation:
  • Convert percentages to decimals (e.g., 50.69% as 0.5069)
  • Result: 79.90 amu

Boron Isotope Abundance Calculation

• Average Atomic Mass: 10.81 amu
• Isotopes: B-10 and B-11
• Objective: Calculate relative percent abundance.
• Formula: Similar to above, solve for % abundance.
• Method:
  • Establish equations with x for B-10 and (1-x) for B-11.
  • Solve equations to find x = 0.20 (B-10) and 1-x = 0.80 (B-11).

Molar Mass Calculations

• Definition: Mass in grams of one mole of a compound.
• Examples
  • Potassium Fluoride (KF):
    • Atomic masses: K = 39.1, F = 19
    • Molar mass = 58.1 g/mol
  • Sodium Carbonate (Na₂CO₃):
    • Atomic masses: Na = 22.99, C = 12.01, O = 16
    • Molar mass = 105.99 g/mol
  • Aluminum Sulfate (Al₂(SO₄)₃):
    • Atomic masses: Al = 26.98, S = 32.06, O = 16
    • Molar mass = 342.14 g/mol

Conversion Between Moles, Grams, and Atoms

• Concept of a Mole: 1 mole = 6.022 x 10²³ entities (Avogadro's Number)
• Conversion Examples:
  • Methane (CH₄):
    • Convert 72g to moles using molar mass.
  • NO₂:
    • Convert 2.5 moles to grams.
  • Carbon and Nickel:
    • Convert atoms to moles and vice versa.

Multi-step Conversions

• Example Problems: Going from grams to atoms or changing substances.
• Process:
  1. Convert grams to moles using molar mass.
  2. Convert moles to atoms using Avogadro's Number.
  3. If needed, adjust for different substances using chemical formulas.

Percent Composition by Mass

• Formula for Element Mass Percent:
  • (Mass of element / Total molar mass of compound) x 100%
• Example with C₆H₁₂O₆ (Glucose):
  • Calculate mass percent for C, H, O.
  • Ensure total percentage sums to 100% for accuracy.

Key Concepts

• Understanding isotopic composition helps determine average atomic masses.
• Molar mass is crucial for converting between moles and grams.
• Avogadro's Number is essential for converting between moles and atoms.
• Accurate calculations require attention to unit conversions and significant figures.

These notes cover the essential steps and concepts for performing calculations involving isotopes, moles, and mass percentages in chemistry.