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Understanding Moles and Atomic Mass

Sep 17, 2024

Calculating Moles of an Element

Introduction

  • Topic for higher tier only
  • Aim: Calculate the number of moles of an element
  • Understanding that concepts may be tricky
  • Importance of reviewing video examples

Relative Atomic Mass

  • Defined as the mass of a single atom of an element
  • Calculated from average masses of isotopes, weighted for abundance
  • Found on the periodic table; provided in exams
  • Example:
    • Carbon: relative atomic mass = 12
    • Oxygen: relative atomic mass = 16
  • Note: Relative atomic mass has no units

The Concept of Moles

  • A mole represents a very large number (Avogadro's number)
  • One mole corresponds to the relative atomic mass of an element in grams
  • Example:
    • 12 grams of carbon = 1 mole of carbon atoms
    • 16 grams of oxygen = 1 mole of oxygen atoms
  • General rule: mass of an element in grams = one mole of atoms of that element
  • Useful for calculating the number of atoms in a chemical reaction

Calculating Number of Moles

  • Formula:
    • Number of moles = Mass (g) / Relative atomic mass
  • Importance of learning this equation

Example Calculations

  1. Magnesium Calculation

    • Mass given: 48 grams
    • Relative atomic mass of magnesium: 24
    • Calculation:
      • Moles = 48 g / 24 = 2 moles
  2. Calcium Calculation

    • Mass given: 120 grams
    • Relative atomic mass of calcium: 40
    • Calculation:
      • Moles = 120 g / 40 = 3 moles
  3. Iron Calculation

    • Mass given: 252 grams
    • Relative atomic mass of iron: 56
    • Calculation:
      • Moles = 252 g / 56 = 4.5 moles
  4. Sulfur Calculation

    • Mass given: 4064 grams
    • Relative atomic mass of sulfur: 32
    • Calculation:
      • Moles = 4064 g / 32 = 127 moles

Conclusion

  • Mole calculations can be manageable with practice
  • More examples available in revision workbook
  • Encouragement to practice and learn