Lecture Notes: Relative Formula Mass and Calculations

Introduction

• Relative Formula Mass (Mr): Average mass of a compound containing various elements.
• Mass Number: Located in the top left corner of an element's nuclear symbol.
  • Includes protons + neutrons.

Relative Atomic Mass (Ar)

• Definition: Average mass of all isotopes of an element.
• Example: Chlorine
  • Isotopes have mass numbers of 35 or 37.
  • Chlorine's relative atomic mass = 35.5 (average mass of isotopes).

Calculating Relative Formula Mass (Mr)

• Process: Add relative atomic masses of all atoms in the compound's molecular formula.
• Example 1: Magnesium Chloride (MgCl₂)
  • Magnesium (Mg) Ar = 24.
  • Chlorine (Cl) Ar = 35.5.
  • Calculation: 24 + (35.5 x 2) = 95.
• Example 2: Sulfuric Acid (H₂SO₄)
  • Hydrogen (H) Ar = 1.
  • Sulfur (S) Ar = 32.
  • Oxygen (O) Ar = 16.
  • Calculation: (2 x 1) + (1 x 32) + (4 x 16) = 98.

Percentage Mass of an Element in a Compound

• Formula:
  • [ \text{Percentage Mass} = \frac{\text{Ar of element} \times \text{Number of atoms}}{\text{Mr of compound}} \times 100 ]
• Example: Sulfur in Sulfuric Acid (H₂SO₄)
  • Sulfur Ar = 32.
  • Calculation: [ \frac{32 \times 1}{98} \times 100 = 32.7% ]

Example: Oxygen in Calcium Hydroxide (Ca(OH)₂)

• Find Mr:
  • Calcium (Ca) Ar = 40.
  • Oxygen (O) Ar = 16.
  • Hydrogen (H) Ar = 1.
  • Calculation for Mr: 40 + (16 x 2) + (1 x 2) = 74.
• Calculate Percentage Mass of Oxygen:
  • Calculation: [ \frac{16 \times 2}{74} \times 100 = 43.2% ]

Conclusion

• Reviewed the process of calculating relative formula mass and percentage mass of elements within compounds.
• Applied these calculations to practical examples like magnesium chloride and sulfuric acid.

End of notes. Hope you enjoyed the session and see you next time!