Lecture Notes: Relative Formula Mass and Calculations

Key Concepts

• Mass Number

  • Found in the top left corner of an element's nuclear symbol.
  • Represents the sum of protons and neutrons in the nucleus.

• Relative Atomic Mass (Ar)

  • The average mass of all isotopes of an element.
  • Denoted by Ar.
  • Example: Chlorine can have mass numbers 35 or 37, but its relative atomic mass is 35.5 due to isotope abundance.

Relative Formula Mass (Mr)

• Used to find the average mass of a compound.
• Calculated by adding the relative atomic masses of all atoms in a compound's molecular formula.

Example Calculations

1. Magnesium Chloride (MgCl₂)

   • Formula: MgCl₂
   • Calculation:
     • 1 Magnesium (Ar = 24)
     • 2 Chlorines (Ar = 35.5 each)
     • Mr = 24 + (35.5 x 2) = 95

2. Sulfuric Acid (H₂SO₄)

   • Formula: H₂SO₄
   • Calculation:
     • 2 Hydrogens (Ar = 1 each)
     • 1 Sulfur (Ar = 32)
     • 4 Oxygens (Ar = 16 each)
     • Mr = (2 x 1) + (1 x 32) + (4 x 16) = 98

Percentage Mass of an Element in a Compound

• Formula:
  • [(\text{Ar of element} \times \text{number of atoms}) / \text{Mr of compound} \times 100]

Example Calculations

1. Sulfur in Sulfuric Acid (H₂SO₄)

   • Calculation:
     • Ar of Sulfur = 32
     • Mr of H₂SO₄ = 98
     • Percentage = [(32 x 1) / 98] x 100 = 32.7%

2. Oxygen in Calcium Hydroxide (Ca(OH)₂)

   • Formula: Ca(OH)₂
   • Relative Atomic Masses:
     • Calcium (Ca) = 40
     • Oxygen (O) = 16
     • Hydrogen (H) = 1
   • Calculation:
     • Oxygen: (16 x 2)
     • Mr: 40 (Ca) + (2 x 16) (O) + (2 x 1) (H) = 74
     • Percentage = [(32) / 74] x 100 = 43.2%

Conclusion

• Understanding the concepts of relative atomic mass and relative formula mass is crucial for calculating the average mass of compounds and the percentage composition of elements within them.