Stoichiometry Tutorial

Definition and Basics

• Stoichiometry: Study of quantitative relationships or ratios between reactants and products in chemical reactions.
• Understanding stoichiometry involves:
  • Coefficients as representations of specific amounts or ratios of reactants and products.

Example Reaction

• Reaction: Hydrogen (H₂) + Nitrogen (N₂) → Ammonia (NH₃)
• Coefficients:
  • 3 H₂ + 1 N₂ → 2 NH₃
  • Represents a ratio of 3:1:2 (hydrogen:nitrogen:ammonia)
  • Ratio consistency: Works for any amount, e.g., 9 H₂ with 3 N₂ produces 6 NH₃.

Ratios and Molar Amounts

• Coefficients represent molar ratios:
  • 3 moles H₂ + 1 mole N₂ → 2 moles NH₃
• Coefficients can represent any amounts including molar amounts.

Mole-Mole Calculations

• Example 1:
  • Given 7.5 moles H₂, find moles of N₂.
  • Use ratio: 3 H₂ : 1 N₂ → 7.5 H₂ * (1 N₂ / 3 H₂) = 2.5 moles N₂
• Example 2:
  • Given 0.8 moles NH₃, find moles of H₂.
  • Use ratio: 3 H₂ : 2 NH₃ → 0.8 NH₃ * (3 H₂ / 2 NH₃) = 1.2 moles H₂

Mass-Mass Calculations

• Conversion Process:
  • Convert mass to moles using molar mass.
  • Use mole ratio for conversion between substances.
  • Convert moles to mass of desired substance.
• Example:
  • Given 42 grams N₂, find mass of NH₃ produced.
  • Steps: 42 g N₂ → 1.5 moles N₂ → 3 moles NH₃ → 51 g NH₃
• Ensure units cancel correctly to validate setup.

Complex Stoichiometric Calculations

• Use of a Map to guide conversions:
  • Mass → Moles → Moles → Mass
  • Conversion with Avogadro's number for particle-mole conversions.
  • Use gas volume conversions at STP: 22.4 L/mole.
• Example:
  • Given 2.8 x 10²⁴ molecules of C₂H₆, find mass of H₂O produced.
  • Path: Molecules → Moles C₂H₆ → Moles H₂O → Mass H₂O → Result: 251 g H₂O

Conclusion

• Stoichiometry provides a systematic approach to calculate quantities in reactions.
• Central to this process are mole-mole conversions and using coefficients as ratios.
• Applicable to various types of problems including mass-mass, mass-volume, and particles-to-mass conversions.