Lecture on Relative Formula Mass

Introduction

• Objective: Understand and calculate relative formula mass for a compound.
• Key Concepts:
  • Relative atomic mass (RAM)
  • Relative formula mass (RFM)

Relative Atomic Mass (RAM)

• Definition: Average mass of isotopes of an element, weighted by abundance.
• Example: Chlorine
  • Two main isotopes: Chlorine-35 and Chlorine-37
  • Both have atomic number 17 (17 protons)
  • Chlorine-35 has 18 neutrons (mass number 35)
  • Chlorine-37 has 20 neutrons (mass number 37)
  • Weighted average mass number: 35.5 (more Chlorine-35 is present)

Relative Formula Mass (RFM)

• Definition: Sum of relative atomic masses of atoms in a compound's formula.
• Symbol: Often represented with M_r
• Key Facts:
  • RFM has no units.
  • RFM calculation does not involve coefficients (big numbers in front of formulas).

Calculating Relative Formula Mass

• Example 1: Methane (CH₄)

  • Formula: 1 carbon atom + 4 hydrogen atoms
  • Carbon RAM: 12
  • Hydrogen RAM: 1
  • RFM Calculation: (1 × 12) + (4 × 1) = 16

• Example 2: Calcium Sulfate (CaSO₄)

  • Formula: 1 calcium atom, 1 sulfur atom, 4 oxygen atoms
  • Calcium RAM: 40
  • Sulfur RAM: 32
  • Oxygen RAM: 16
  • RFM Calculation: (1 × 40) + (1 × 32) + (4 × 16) = 136

• Example 3: Magnesium Hydroxide (Mg(OH)₂)

  • Formula: 1 magnesium atom, 2 oxygen atoms, 2 hydrogen atoms
  • Magnesium RAM: 24
  • Oxygen RAM: 16
  • Hydrogen RAM: 1
  • RFM Calculation: (1 × 24) + (2 × 16) + (2 × 1) = 58

Additional Resources

• Study Material: Vision workbook with more problems on calculating RFM.

Conclusion

• After this session, you should be able to:
  • Define relative formula mass.
  • Calculate relative formula mass for simple compounds.

[End of Lecture]