Chemistry Lecture on Moles, Particles, and Avogadro's Number

Key Concepts

Variables and Formulas

• n (small): Number of moles
• N (capital): Number of atoms, particles, or molecules
• N_A: Avogadro's Number
  • 6.02 x 10²³ (provided in exams)

Important Formula

• N = n x N_A
  • N is the number of particles
  • n is the number of moles
  • N_A is Avogadro's Number

Methods Using the Formula

1. Triangular Representation: Helps to use the formula in various ways
   • N at the top
   • n and N_A at the bottom

Working Examples

Example 1

Determine the number of moles if you have 12.04 x 10²³ particles

• Given:
  • Number of particles, N = 12.04 x 10²³
  • N_A = 6.02 x 10²³
• Applying the formula:
  • n = N / N_A
  • n = (12.04 x 10²³) / (6.02 x 10²³)
  • Result: n = 2 moles

Example 2

Determine the number of molecules in 3 moles of H₂O

• Given:
  • Number of moles, n = 3
  • N_A = 6.02 x 10²³
• Applying the formula:
  • N = n x N_A
  • N = 3 x 6.02 x 10²³
  • Result: N = 1.806 x 10²⁴ molecules

Complex Calculations with Different Data

Example with Grams

Determine the number of molecules in 36 grams of H₂O

• Step 1: Calculate moles

  • n = given mass / molar mass
  • Given mass = 36g, Molar mass of H₂O = 18 g/mol
  • n = 36g / 18 g/mol = 2 moles

• Step 2: Calculate number of molecules using moles

  • N = n x N_A
  • N = 2 x 6.02 x 10²³
  • Result: N = 1.204 x 10<sup>24 molecules

Practical Application with Compounds

Example: Water Molecules

• 10 H₂O molecules have:
  • 10 oxygen atoms
  • 20 hydrogen atoms
  • 30 atoms altogether

Example: Carbon Dioxide (CO₂) Molecules

• 5 CO₂ molecules have:
  • 5 carbon atoms
  • 10 oxygen atoms
  • 15 atoms altogether

Advanced Mole Calculations

Example with CaCO₃

• Given: 2 moles of CaCO₃

1. Number of CaCO₃ molecules

• N = n x N_A
  • N = 2 x 6.02 x 10²³
  • Result: N = 1.204 x 10<sup>24 molecules

2. Number of atoms in CaCO₃

• Ca atoms: 1 atom per molecule
  • 1.204 x 10<sup>24
  • C atoms: 1 atom per molecule
    • 1.204 x 10<sup>24
  • O atoms: 3 atoms per molecule
    • 3 x 1.204 x 10²⁴ = 3.612 x 10<sup>24

Summary

• The ability to rearrange and understand chemistry formulas is crucial.
• Utilizing Avogadro's Number and understanding moles are foundational skills in chemistry.