What is Stoichiometry

• A way of analyzing the process that occur within a chemical reaction

• Comes from two greek words

  • stoicheion = Element

  • Metron = measure

• Two main type

  • Composition Stoichiometry- deals with mass relationships of elements in compounds (We already know how to do this)

  • Reaction Stoichiometry- Deals in the mass relationships between reactants and products in a reaction

• Based on the law of conservation of mass

Why Even Bother with Stoichiometry

• Stoichiometry allows us to know how much we need of a particular reactant to get the amount of product we want

  • Analogy: Lets say you are making Turkey Sandwiches for a party

    • Each sandwich needs

      • 2 pieces of bread

      • 3 pieces of cheese

      • 1 piece of lettuce

      • 4 Slices of Turkey

      • 3 Teaspoons of mayo​

    • If you need 150 sandwiches, how much cheese do you need? What if you are making 200?

    • Chemical Reactions work the same way

Reaction Stoichiometry Problems

• Classified into four groups depending on what information you are given and what you are expected to find (the unknown)

  • can be any combination of reactant and product

  • Masses are typically expressed in grams, but not always

  • Ratios from balanced equations are used

Problem Type 1: Given and Unknown are in moles

• You are given a quantity of a substance in Moles and asked to solve for an unknown quantity in moles

• Example: Ammonia is made industrially by reacting nitrogen and hydrogen under pressure, at high temperature, and in the presence of a catalyst. The equation is N2(g) + 3H₂(g)→2NH₃(g). If 4.0 mol of H₂ react, how many moles of NH₃ will be produced?

  • What information are we given?

    • Balanced equation, 4.0 moles of H₂​

Problem Type 1: Given and Unknown are in moles

• Ammonia is made industrially by reacting nitrogen and hydrogen under pressure, at high temperature, and in the presence of a catalyst. The equation is N2(g) + 3H₂(g)→2NH₃(g). If 4.0 mol of H₂ react, how many moles of NH₃ will be produced?

  • What information are we given?

    • Balanced equation, 4.0 moles of H₂

    • How do we set up?

      • Identify the ratio of H₂ to NH₃

        • 3 H₂ to 2NH₃​

Problem Type 1: Given and Unknown are in moles

Problem Type 1: Given and Unknown are in moles

Problem Type 2: Given is in moles and Unknown is a mass

Problem Type 2: Given is in moles and Unknown is a mass