Fundamentals of Stoichiometry

Sep 1, 2024

Basic Introduction to Stoichiometry

Definition

  • Stoichiometry: Calculation of products and reactants in a chemical reaction.
    • Strych means element
    • Metry means measurement

Importance of Coefficients in Chemical Reactions

  • Coefficients are used to balance chemical reactions.
    • Example: Reaction of Hydrogen (H2) and Nitrogen (N2) to form Ammonia (NH3).
    • Balanced Reaction: 3 H2 + 1 N2 → 2 NH3
  • Coefficients Represent:
    1. Amount of Substances:
      • 3 (H2) = 3 moles of hydrogen gas
      • 1 (N2) = 1 mole of nitrogen gas
      • 2 (NH3) = 2 moles of ammonia
    2. Ratio of Reactants to Products:
      • Ratio = 3 H2 : 1 N2 : 2 NH3
    3. Number of Molecules:
      • 3 molecules of H2, 1 molecule of N2, 2 molecules of NH3

Using Coefficients for Calculations

  • Example Problem: Forming 6 molecules of NH3.
    • Need 9 molecules of H2 and 3 molecules of N2.
    • Ratio used: 3 H2 : 1 N2 : 2 NH3

Stoichiometric Conversions

1. Mole to Mole Conversion

  • Example Problem: Moles of N2 needed for 13.5 moles of H2.
    • Reaction: 3 H2 + 1 N2 → 2 NH3
    • Given: 13.5 moles of H2
    • Calculation:
      • Ratio: 3 H2 : 1 N2
      • Cross multiply: 3x = 13.5
      • Result: 4.5 moles of N2 needed.

2. Mole to Gram Conversion

  • Example Problem: Grams of CO2 formed from 4 moles of C3H8 (propane).
    • Reaction: C3H8 + O2 → CO2 + H2O
    • Balanced Equation:
      • 1 C3H8 : 3 CO2
    • Calculation:
      • 4 moles of C3H8 produce 12 moles of CO2
      • Molar mass of CO2 = 44 g
      • Convert: 12 moles × 44 g = 528 g of CO2.

3. Grams to Mole Conversion

  • Example Problem: Moles of H2 for 6 g of N2.
    • Reaction: H2 + N2 → NH3
    • Given: 6 g of N2; Molar mass = 28 g
    • Calculation:
      • Convert to moles: 6 g / 28 g = 0.21 moles of N2
      • Ratio: 3 H2 : 1 N2
      • Result: 0.69 moles of H2.

4. Grams to Grams Conversion

  • Example Problem: Grams of O2 for 10 g of H2.
    • Reaction: H2 + O2 → H2O
    • Molar Mass: H2 = 2 g, O2 = 32 g
    • Calculation:
      • Convert 10 g H2 to moles: 10 g / 2 g = 5 moles of H2
      • Ratio: 2 H2 : 1 O2
      • Result: 2.5 moles of O2 = 80 g O2.

Conclusion

  • Key Steps in Stoichiometry Problems:
    1. Convert mass to moles of known species.
    2. Convert number of moles to unknown species.
    3. Convert number of moles to grams.
  • Mastering these conversions is crucial for stoichiometry.