Introduction to Stoichiometry

Definition

• Stoichiometry is the calculation of products and reactants in a chemical reaction.
  • "Stoichio" means element, "metry" means measurement.
  • It helps find the number of moles of reactants and products.

Coefficients in Chemical Reactions

• Coefficients are numbers placed before compounds in a chemical equation.
  • Example Reaction: Hydrogen gas reacts with nitrogen gas to form ammonia (NH3).
  • Balanced Reaction: 3H2 + N2 → 2NH3
  • Coefficients:
    • 3 for H2 (hydrogen gas)
    • 1 for N2 (nitrogen gas)
    • 2 for NH3 (ammonia)

Stories of Coefficients

1. Number of Moles: Coefficient indicates moles of a substance.
   • 3 moles of H2, 1 mole of N2, 2 moles of NH3.
2. Ratio of Reactants to Products:
   • Ratio of H2 to N2 to NH3 is 3:1:2.
3. Number of Molecules:
   • Coefficient also indicates molecules, e.g., 3 molecules of H2.

Stoichiometric Calculations

Mole-to-Mole Conversion

• Problem: How many moles of N2 needed for 13.5 moles of H2 to form NH3?
  • Balanced Reaction: 3H2 + N2 → 2NH3
  • Solution:
    • Given 13.5 moles of H2.
    • Ratio of H2 to N2 is 3:1.
    • Cross-multiply: 3x = 13.5
    • Solve for x: x = 4.5 moles of N2 needed.

Mole-to-Gram Conversion

• Problem: Moles of sulfur trioxide from 8.5 moles of sulfur dioxide?
  • Reaction: 2SO2 + O2 → 2SO3
  • Solution:
    • Ratio of SO2 to SO3 is 2:2.
    • Given 8.5 moles of SO2, cross-multiply to find SO3.
    • Result: 8.5 moles of SO3 can form.

Grams-to-Mole Conversion

• Problem: Moles of H2 needed for 6g of N2 to produce NH3?
  • Reaction: 3H2 + N2 → 2NH3
  • Solution:
    • Convert 6g N2 to moles: 6g / 28g/mol = 0.23 moles N2.
    • Ratio H2 to N2 is 3:1.
    • Cross-multiply to find H2: 0.69 moles of H2 needed.

Grams-to-Grams Conversion

• Problem: Grams of O2 needed for 10g of H2 to form H2O?
  • Reaction: 2H2 + O2 → 2H2O
  • Solution:
    • 10g H2 = 5 moles.
    • Ratio H2 to O2 is 2:1.
    • Cross-multiply to find O2: 2.5 moles O2.
    • Convert moles O2 to grams: 2.5 moles × 32g/mol = 80g O2.

Conclusion

• Stoichiometry involves conversions between moles and grams.
• Understanding coefficients, ratios, and conversions are crucial to solving stoichiometric problems.