Tutorial on Stoichiometry

What is Stoichiometry?

• Definition: Stoichiometry involves understanding the quantitative relationships in chemical reactions.
• Key Question: How do the quantities written in a chemical equation relate mathematically?

Simple Reaction Example

• Example Reaction: Hydrogen (H₂) reacting with Nitrogen (N₂) to produce Ammonia (NH₃)
• Balancing the Equation: Adding coefficients to balance chemical equations.

Key Concepts in Stoichiometry

• Coefficients: Represent the number of particles or the ratio of reactants to products.
• Understanding Coefficients: Determines how many particles are reacting and how many are produced.

Particle Ratios

• Example: 3 H₂ molecules react with 1 N₂ molecule to produce 2 NH₃ molecules.
• Example Scaling:
  • 6 H₂ will react with 2 N₂
  • 9 H₂ will react with 3 N₂
  • Always maintains a 3:1:2 ratio (H₂:N₂:NH₃)

Molar Ratios

• Understanding Mole: Coefficients represent molar ratios.
• Example: 3 moles of H₂ react with 1 mole of N₂ to produce 2 moles of NH₃
• Usefulness: Helps relate any amount of one substance to another in the reaction.

Solving Stoichiometry Problems

Mole-to-Mole Conversion

• Example: 7.5 moles of H₂ -> How many moles of N₂?
  • Use 3:1 ratio of H₂ to N₂
  • Calculation: 7.5 moles H₂ x (1 mole N₂ / 3 moles H₂) = 2.5 moles N₂
• Example: How much H₂ needed to get 0.8 moles of NH₃?
  • Use 3:2 ratio of H₂ to NH₃
  • Calculation: 0.8 moles NH₃ x (3 moles H₂ / 2 moles NH₃) = 1.2 moles H₂

Mole-Mass Conversion

• Process Map:

  1. Convert mass given to moles
  2. Use mole-to-mole ratio to switch substances
  3. Convert moles of the new substance back to mass

• Example: What mass of NH₃ from 42g of N₂?

  • Step 1: Convert 42g N₂ to moles: 42g x (1 mole N₂ / 28g N₂) = 1.5 moles N₂
  • Step 2: Convert moles N₂ to moles NH₃: 1.5 moles N₂ x (2 moles NH₃ / 1 mole N₂) = 3 moles NH₃
  • Step 3: Convert to mass: 3 moles NH₃ x (17g NH₃ / 1 mole NH₃) = 51g NH₃

Additional Examples

• Example: Mass of O₂ needed to react with 5.95g NH₃?
  • Step 1: Convert 5.95g NH₃ to moles: 5.95g x (1 mole NH₃ / 17g NH₃) = 0.35 moles NH₃
  • Step 2: Use ratio 7:4 (O₂:NH₃): 0.35 moles NH₃ x (7 moles O₂ / 4 moles NH₃) = 0.61 moles O₂
  • Step 3: Convert to mass: 0.61 moles O₂ x (32g O₂ / 1 mole O₂) = 19.6g O₂

Combustion Reaction of Ethane (C₂H₆)

• Steps:
  • Convert given mass to moles
  • Use mole-to-mole conversion
  • Convert desired moles to mass
• Example: Reaction of 37.5g C₂H₆ to CO₂?
  • Step 1: Convert 37.5g C₂H₆ to moles
  • Step 2: Use ratio
  • Step 3: Convert to mass
  • Result: 110g CO₂

Extending Stoichiometric Calculations

• Using Particle Counts: Avogadro's number: 6.02 x 10²³ particles = 1 mole

  • Example: Converting particles to moles or vice versa

• Gas Volume Conversion: 1 mole of gas = 22.4 L at STP

  • Example: Volume to particles or mass

• Complex Example: Mass of water produced from given molecules of C₂H₆

  • Process: Particles -> Moles -> Moles -> Mass
  • Calculation: 2.8 x 10²⁴ molecules C₂H₆ -> 251g Water

Key Takeaways

• Central Calculation: Always involves mole-to-mole conversion.
• Dimensional Analysis: Ensure units cancel out correctly.
• Practical Usefulness: Predicts product amounts or reactant needs.

Practice Problems

• Couple more problems to solidify understanding.
• Extending beyond mass-to-mass calculations.