Lecture Notes: Relative Formula Mass and Calculations

Key Concepts

• Mass Number: Found in the top left corner of an element's nuclear symbol; equals number of protons + neutrons.
• Relative Atomic Mass (Ar): The average mass of all isotopes of an element, considering their abundance.
  • Example: Chlorine can have mass numbers 35 or 37 but has an Ar of 35.5.

Relative Formula Mass (Mr)

• Definition: The sum of the relative atomic masses of all atoms in a compound's molecular formula.
• Calculation:
  • Magnesium Chloride (MgCl₂):
    • Mg: Ar = 24
    • Cl: Ar = 35.5
    • Mr = 24 + 2(35.5) = 95
  • Sulfuric Acid (H₂SO₄):
    • H: Ar = 1
    • S: Ar = 32
    • O: Ar = 16
    • Mr = 2(1) + 32 + 4(16) = 98

Percentage Mass Calculation

• Formula: ( \text{Percentage mass} = \frac{\text{Ar of element} \times \text{number of atoms}}{\text{Mr of compound}} \times 100 )
• Example Calculations:
  • Sulfur in Sulfuric Acid (H₂SO₄):
    • Ar of S = 32
    • Mr of H₂SO₄ = 98
    • Percentage mass = ( \frac{32 \times 1}{98} \times 100 = 32.7% )
  • Oxygen in Calcium Hydroxide (Ca(OH)₂):
    • Ar of O = 16
    • Mr of Ca(OH)₂ = 40 + 2(16) + 2(1) = 74
    • Percentage mass = ( \frac{32}{74} \times 100 = 43.2% )

Summary

• Understanding relative formula mass and percentage mass calculations is crucial for analyzing compounds.
• Practice with different compounds to become comfortable with these calculations.

End of notes for the video lecture on relative formula mass and calculations.