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Homework Help: Avogadro's Theory

  1. Oct 24, 2007 #1
    1. The problem statement, all variables and given/known data
    This question might sound confusing, because my teacher said it only once, and I don't think I have it down right. So if the question is confusing but you know what I'am trying to get at, please say so. The question is: According to Avogadro's Theory, how are the number of molecules and atoms related in a chemical equation?

    2. Relevant equations
    I don't think equations are necessary.

    3. The attempt at a solution
    I think that the number of molecules would be the same as the number of atoms in a chemical equation, but I'am not sure.
  2. jcsd
  3. Oct 24, 2007 #2
    I think I understand your question

    Avogadro's number = 6.022 x 10^23 "things" = 1 mole

    1.the "things" may be ATOMS if the substance is an ELEMENT

    so 6.022 x 10^23 "ATOMS" = 1 mole of an ELEMENT

    2. the "things" may be MOLECULES if the substance is a COMPOUND (molecular to be precise)

    so 6.022 x 10^23 "MOLECULES" = 1 mole of an COMPOUND

    For the equation:

    Si + O2 ---> SiO2

    1 mol Si + 1 mol O2 ---> 1 mol SiO2

    6.022 x 10^23 "ATOMS" Si + 6.022 x 10^23 "MOLECULES" O2 ---> 6.022 x 10^23 "MOLECULES" SiO2

    If this is not what you mean, then pls ask again, it may be related to the other aspect of Avogadro's principle of gases and equal volumes of gases = equal volumes of moles

    (my example is just to illustrate the point, even though also SiO2 really exists as a network solid so its stretching it a bit to think of it as a single molecule)
  4. Oct 24, 2007 #3
    Thanks for the explanation. You cleared up a lot of the confusion I had. Can you also explain the other aspect of Avogadro's principle of gases and equal volumes of gases = equal volumes of moles that you mentioned please?
  5. Oct 24, 2007 #4
    glad you got the first part

    Avogadro's principle (this he really did - Avogadro's number he never calculated - they just honored him with it since the Principle led others to the huge number)

    Comparing gases AT THE SAME TEMP AND PRESSURE (same conditions - not necessarily at STP) in a reaction mixture...

    - the volume of Gas A will have the same number of molecules (or can say moles) of Gas B if Gas B has the with the same volume as Gas A

    Classic example:

    H2 + Cl2 ---> 2HCl

    coefficients 1 1 2
    volumes (eg. in L) 1L 1L 2L
    molecules/mole 1 1 2

    or N2 + 3H2 ---> 2NH3

    coefficients 1 3 2
    volumes (eg. in L) 1L 3L 2L
    molecules/mole 1 3 2

    so we can predict the volumes of gases using stoichiometry

    just remember this works ONLY if the gases are under the same conditions
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