Lecture Notes: Stoichiometry and Mass Conversion in Chemical Reactions

Key Concepts

• Stoichiometric Coefficients: Used to relate the moles of one substance to another in a chemical reaction.
• Conversion Pathways: Ability to convert between moles, molecules, and mass using:
  • Avogadro's number (moles to molecules)
  • Molar mass (moles to mass)

Mass to Mass Calculation in Chemical Reactions

• Objective: Convert the mass of a reactant to the mass of a product.
• Example: Methane combustion to water.

Example 1: Methane Combustion

1. Start with Mass of Methane:
   • 1 gram of methane (CH₄) combusts completely.
2. Convert Mass to Moles:
   • Use molar mass of methane: 16 g/mol
   • Calculation: (\frac{1 \text{ g}}{16 \text{ g/mol}} = 0.0625 \text{ moles of methane})
3. Use Stoichiometric Ratio:
   • Methane to water: 1 mole of CH₄ produces 2 moles of H₂O
   • Calculation: (0.0625 \text{ mol CH₄} \times 2 = 0.125 \text{ moles of H₂O})
4. Convert Moles of Water to Mass:
   • Use molar mass of water: 18 g/mol
   • Calculation: (0.125 \text{ mol H₂O} \times 18 \text{ g/mol} = 2.25 \text{ grams of water})

Example 2: Hexane Combustion

1. Starting Point:
   • 5 grams of hexane (C₆H₁₄)
2. Convert Mass to Moles of Hexane:
   • Use molar mass of hexane: (\approx 86 \text{ g/mol})
3. Stoichiometric Ratio:
   • 2 moles of hexane produce 10 moles of water
   • Multiply moles of hexane by 5 to get moles of water
4. Convert Moles of Water to Mass:
   • Use molar mass of water to find final mass of water.

Summary

• Process Summary:
  1. Convert mass of reactant to moles.
  2. Use stoichiometric ratios to find moles of product.
  3. Convert moles of product to mass.
• Application: These steps can be applied to any chemical reaction to convert from mass of reactants to mass of products.