Introduction to Stoichiometry

Types of Conversions

1. Moles of Substance A to Moles of Substance B
   • Identify the mole ratio.
   • Single-step conversion.
   • Example: Convert moles of sulfur dioxide (SO2) to moles of sulfur trioxide (SO3).
2. Moles of Substance A to Grams of Substance B
   • Two-step conversion.
   • First, convert moles of A to moles of B.
   • Then, convert moles of B to grams using molar mass.
   • Example: Convert moles of propane (C3H8) to grams of carbon dioxide (CO2).
3. Grams of Substance A to Grams of Substance B
   • Three-step conversion.
   • First, convert grams of A to moles of A.
   • Then, convert moles of A to moles of B.
   • Finally, convert moles of B to grams using molar mass.
   • Example: Convert grams of aluminum (Al) to grams of aluminum chloride (AlCl3).

Detailed Examples

Example 1: Sulfur Dioxide to Sulfur Trioxide

• Balanced Equation: 2 SO2 + O2 → 2 SO3
• Given: 3.4 moles of SO2
• Required: Moles of SO3
  • Use mole ratio 2:2.
  • Calculation: 3.4 moles SO2 × (2 moles SO3 / 2 moles SO2) = 3.4 moles SO3

Example 2: Sulfur Dioxide to Oxygen

• Given: 4.7 moles of SO2
• Required: Moles of O2
  • Use mole ratio 2:1.
  • Calculation: 4.7 moles SO2 × (1 mole O2 / 2 moles SO2) = 2.35 moles O2

Example 3: Propane to Carbon Dioxide

• Balanced Equation: C3H8 + 5 O2 → 3 CO2 + 4 H2O
• Given: 2.8 moles of C3H8
• Required: Grams of CO2
  • Step 1: Convert moles of C3H8 to moles of CO2 using mole ratio 1:3.
    • 2.8 moles C3H8 × (3 moles CO2 / 1 mole C3H8) = 8.4 moles CO2
  • Step 2: Convert moles of CO2 to grams using molar mass of CO2 (44.01 g/mol).
    • 8.4 moles CO2 × 44.01 g/mol = 369.48 grams CO2

Example 4: Propane to Oxygen

• Given: 3.8 moles of C3H8
• Required: Grams of O2
  • Step 1: Convert moles of C3H8 to moles of O2 using mole ratio 1:5.
    • 3.8 moles C3H8 × (5 moles O2 / 1 mole C3H8) = 19.0 moles O2
  • Step 2: Convert moles of O2 to grams using molar mass of O2 (32 g/mol).
    • 19.0 moles O2 × 32 g/mol = 608 grams O2

Example 5: Propane to Water

• Given: 25 grams of C3H8
• Required: Moles of H2O
  • Step 1: Convert grams of C3H8 to moles using molar mass (44.09 g/mol).
    • 25 grams C3H8 × (1 mole / 44.09 grams) = 0.567 moles C3H8
  • Step 2: Convert moles of C3H8 to moles of H2O using mole ratio 1:4.
    • 0.567 moles C3H8 × (4 moles H2O / 1 mole C3H8) = 2.27 moles H2O

Example 6: Water to Carbon Dioxide

• Given: 38 grams of H2O
• Required: Moles of CO2
  • Step 1: Convert grams of H2O to moles using molar mass (18.016 g/mol).
    • 38 grams H2O × (1 mole / 18.016 grams) = 2.11 moles H2O
  • Step 2: Convert moles of H2O to moles of CO2 using mole ratio 4:3.
    • 2.11 moles H2O × (3 moles CO2 / 4 moles H2O) = 1.58 moles CO2

Example 7: Aluminum to Aluminum Chloride

• Balanced Equation: 2 Al + 3 Cl2 → 2 AlCl3
• Given: 35 grams of Al
• Required: Grams of AlCl3
  • Step 1: Convert grams of Al to moles using molar mass (26.98 g/mol).
    • 35 grams Al × (1 mole / 26.98 grams) = 1.30 moles Al
  • Step 2: Convert moles of Al to moles of AlCl3 using mole ratio 2:2.
    • 1.30 moles Al × (2 moles AlCl3 / 2 moles Al) = 1.30 moles AlCl3
  • Step 3: Convert moles of AlCl3 to grams using molar mass (133.34 g/mol).
    • 1.30 moles AlCl3 × 133.34 g/mol = 173.34 grams AlCl3

Example 8: Aluminum to Chlorine

• Given: 42.8 grams of Al
• Required: Grams of Cl2
  • Step 1: Convert grams of Al to moles using molar mass (26.98 g/mol).
    • 42.8 grams Al × (1 mole / 26.98 grams) = 1.59 moles Al
  • Step 2: Convert moles of Al to moles of Cl2 using mole ratio 2:3.
    • 1.59 moles Al × (3 moles Cl2 / 2 moles Al) = 2.39 moles Cl2
  • Step 3: Convert moles of Cl2 to grams using molar mass (70.9 g/mol).
    • 2.39 moles Cl2 × 70.9 g/mol = 169.45 grams Cl2

Key Takeaways

• Conversion steps depend on the given and required units.
• Always use the balanced chemical equation for accurate mole ratios.
• Use molar masses for conversion between grams and moles.