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Understanding Moles in Chemical Compounds

Apr 9, 2025

Calculating the Number of Moles of a Compound

Introduction

  • Topic: Calculation of moles of a compound (Higher Tier)
  • Objective: Ability to calculate the number of moles of a compound.
  • Previous Knowledge: Understanding of concept of a mole as a large number.
  • Equation for Elements: Number of moles = mass (g) / relative atomic mass

Relative Formula Mass

  • Definition: Sum of relative atomic masses of all atoms in a compound's formula.
  • No Unit: Relative formula mass has no unit.

Formula for Compounds

  • Equation for Compounds:
    • Number of moles = mass given (g) / relative formula mass (M_r)
  • Difference from Elements Formula: Divides by relative formula mass instead of relative atomic mass.

Example Calculations

Example 1: Calcium Carbonate (CaCO3)

  • Given Mass: 300g
  • Atomic Masses:
    • Calcium (Ca): 40
    • Carbon (C): 12
    • Oxygen (O): 16 (3 atoms)
  • Relative Formula Mass Calculation:
    • M_r = 40 + 12 + 3(16) = 100
  • Moles Calculation:
    • Number of moles = 300 / 100 = 3 moles

Example 2: Magnesium Chloride (MgCl2)

  • Given Mass: 380g
  • Atomic Masses:
    • Magnesium (Mg): 24
    • Chlorine (Cl): 35.5 (2 atoms)
  • Relative Formula Mass Calculation:
    • M_r = 24 + 2(35.5) = 95
  • Moles Calculation:
    • Number of moles = 380 / 95 = 4 moles

Example 3: Lithium Sulfate (Li2SO4)

  • Given Mass: 990g
  • Atomic Masses:
    • Lithium (Li): 7 (2 atoms)
    • Sulfur (S): 32
    • Oxygen (O): 16 (4 atoms)
  • Relative Formula Mass Calculation:
    • M_r = 2(7) + 32 + 4(16) = 110
  • Moles Calculation:
    • Number of moles = 990 / 110 = 9 moles

Example 4: Burum Hydroxide (Be(OH)2)

  • Given Mass: 64.5g
  • Atomic Masses:
    • Berum (Be): n (given)
    • Oxygen (O): 16 (2 atoms)
    • Hydrogen (H): 1 (2 atoms)
  • Relative Formula Mass Calculation:
    • M_r = n + 2(16) + 2(1) = 43
  • Moles Calculation:
    • Number of moles = 64.5 / 43 = 1.5 moles

Conclusion

  • Additional Resources: More questions available in a revision workbook linked in the video.
  • Key Learning Outcome: Ability to calculate the number of moles of a compound.

By understanding these concepts and formulas, students can effectively calculate moles in various chemical compounds, an essential skill in chemistry.

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