Lecture Notes: Mole Ratio Problems and Stoichiometry

Introduction

• Importance of mole ratio problems in stoichiometry.
• Two methods for solving mole ratio problems:
  1. Recipe Method
  2. Conversion Factor Method

Recipe Method

• Treats chemical equations like a recipe.
• Requires understanding the "ingredients" (reactants) and "baked goods" (products).
• Steps:
  • Identify the given mole ratio in the chemical equation.
  • When given a different starting amount, find a multiplication factor to adjust the whole equation.
  • Multiply all components by this factor to find the number of moles.
• Example: From 2 mol H2O -> 2 mol H2 + 1 mol O2, starting with 6.2 mol H2O:
  • Multiply all parts by 3.1 to maintain the ratio.
  • Result: 3.1 mol O2 produced.

Conversion Factor Method

• Uses conversion factors derived from the chemical equation.
• Direct computation, but may lack intuitive understanding.
• Steps:
  • Write conversion factors from the equation (e.g., 2 mol H2O : 1 mol O2).
  • Choose the conversion factor that cancels out the starting unit.
  • Multiply to find the desired amount.
• Example: Same equation, starting with 6.2 mol H2O:
  • Use conversion factor (1 mol O2 / 2 mol H2O).
  • Result: 3.1 mol O2 produced.

Further Examples

• Example 2: 19.2 mol O2 needed:
  • Recipe Method: Multiply the initial equation by 19.2.
  • Conversion Factor: Start with 19.2 mol O2, use conversion factor to find required H2O.
• Example 3: Combining 8.4 mol H2S with O2:
  • Recipe Method: Multiply entire equation by 4.2 to scale up from 2 mol H2S.
  • Conversion Factor: Use appropriate conversion factor to solve.
• Example 4: Starting with 9.2 mol O2, calculate H2S needed:
  • Recipe Method: Multiply to adjust from 3 mol O2 to 9.2 mol.
  • Conversion Factor: Convert O2 to H2S using the factor.
• Example 5: Reacting 7.2 mol C3H8 with O2:
  • Recipe Method: Scale up equation by 7.2.
  • Conversion Factor: Use factor to convert propane to oxygen needed.
• Example 6: Making 13.5 mol CO2 from C3H8:
  • Recipe Method: Adjust from 3 mol CO2 to 13.5 mol.
  • Conversion Factor: Apply factor to relate CO2 and C3H8.

Conclusion

• Both methods yield the same results, but understanding both can be beneficial.
• Conversion factors are often favored by textbooks and teachers, but the concept of a 'recipe' helps understand the process intuitively.
• Practice both methods to strengthen your stoichiometry skills.