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    • #25 - Stoichiometry 101

    If I were making cheese sandwiches, I could create a formula like this:

    2B + 1C --> 1CS
    (2 peaces of bread, plus one slice of cheese, makes one cheese sandwich)

    This means for every two slices of bread, I�ll need one slice of cheese. But, you can�t say that you need two pounds of bread for every pound of cheese. Why, because they weigh different. Two pounds of bread(about 2-3 loaves) will be too much bread of just one pound of cheese (about 8-12 slices)!

    Like in this reaction:
    N2 + 3H2 -->2NH3

    You would need one N2, for every 3 H2�s. Also, you�re determined to use just one gram of N2. That�s all your teacher has left of in the lab. So, how would you find out how much H2 you would need to have a reaction with no N2 or H2�s left?

    Believe it our not, you can determine it with just one factor label!

    First, you need to convert that one gram of N2 into moles: 14.01 + 14.01 = 28.02 g/mol

    1 gram of N2 | 1 mole N2_______ | __________________
    | 28.02 grams of N2 |

    (that last unit on the top indicates moles of N2)

    Then, continue the factor label by, converting how many moles of N2 you�ll need with the moles of H2. In the equation above, for every one mole of N2, you�ll need 3 moles of H2, so put that in the equation:

    1 gram of N2 | 1 mole N2_______ | 3 moles H2 | _________
    | 28.02 grams of N2 | 1 mole N2 |

    (This conversion factor is called the mole ratio, it�s basically the coefficients in the equation)

    Then convert those moles of H2 to grams of H2:
    1.008 + 1.008 = 2.016 g/mol

    1 gram of N2 | 1 mole N2_______ | 3 moles H2 | 2.016 grams H2 _________
    | 28.02 grams of N2 | 1 mole N2 | 1 mole H2

    Multiply the top and the bottom:

    1 gram of N2 | 1 mole N2_______ | 3 moles H2 | 2.016 grams H2 __ = 6.048 grams H2
    | 28.02 grams of N2 | 1 mole N2 | 1 mole H2 = 28.02

    = .2158 grams of H2


    You would need .2158 grams of H2 for every one gram of N2 Again as you do this more often, you should get the hang of this method. This type of science is called stoichiometry, where you consider how much of a chemical is consumed or produced in a chemical reaction.
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