Stoichiometry

​Stoichiometry & Stoichiometric Calculations

Objectives

Define stoichiometry.

Describe the importance of the mole ratio in stoichiometric calculations.

Write a mole ratio relating two substances in a chemical equation.

Calculate the amount in moles/mass of a reactant or product from the amount in moles/mass of a different reactant or product.

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 types

  • 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 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: let's 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 of the coefficients of a balanced equations are used

    (this is called the Mole Ratio)

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 by reacting nitrogen and hydrogen. The balanced equation is N_{2 (g)} + 3H_{2 (g)}→2NH_{3 (g)}. If 4.0 mol of H₂ react, how many moles of NH₃ will be produced?

What information are we given?
A balanced equation, and 4.0 moles of H₂

How do we set up?

Identify the Mole Ratio - the ratio of NH₃ (the unknown) to H₂ (the given) :

2NH₃ to 3H₂ the ratio is based on their coefficients

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

​So the four types of possible stoichiometric calculations are:
1. Given in moles (mol G); Wanted in mole (mol W)
2. Given in moles (mol G); Wanted in grams (mass of W)
3. Given in grams (mass of G); Wanted in moles (mol W)
4. Given in grams (mass of G); Wanted in grams (mass of W)