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Stoichiometry Practice Problems

  1. Feb 16, 2006 #1
    Aluminum oxide is formed from the reaction of metallic aluminum with oxygen gas. How many moles of Aluminum are needed to form 3.4 moles of Aluminum oxide?

    I don't necessarily need help solving the problem itself (I want to try to do it on my own) but I don't understand how to set the equation up. Aluminum oxide? --> AlO? Metallic aluminum? --> Al? Oxygen gas? --> O?

    Al + O --> AlO ?

    That seems too easy...and wrong...:confused: :rofl:
     
  2. jcsd
  3. Feb 16, 2006 #2

    Borek

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    Look at the periodic table - Na2O, MgO, SiO2 - what should be the formula of aliminum oxide?
     
  4. Feb 16, 2006 #3
    Sorry for the wait, computer problems. :)

    Anyways, is it...

    Al + O2 --> Al2O3
     
  5. Feb 17, 2006 #4

    Borek

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    Exactly.

    Now it should be a breeze...
     
  6. Feb 17, 2006 #5
    Well, we're talking about me doing this so we'll see...:tongue:

    Is this correctly balanced?

    4Al + 3O2 --> 2Al2O3
     
  7. Feb 18, 2006 #6
    Ah, I know it was supposed to be easy (And I'm sure it is as my teacher said so as well on the lesson page.) but I don't know what to do. I think the problem lies in what it is I'm doing exactly...what is stoichiometry? I read the definitions and took the notes but it's still a bit fuzzy.
     
  8. Feb 18, 2006 #7

    Borek

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    Reaction is balanced. Now - think in terms of proportions. You need 4 moles of Al to obtain 2 moles of oxide. If so, how many moles of Al are needed to prepare 3.4 mole of oxide?
     
  9. Feb 18, 2006 #8
    I can do that but I don't understand why I'm doing it if I need to know how many moles of Aluminum are needed to make 3.4 moles of Aluminum Oxide...:frown:
     
  10. Feb 18, 2006 #9

    Gokul43201

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    Perhaps you haven't thought about what the balanced equation is telling you. How would you put the equation into words ?
     
  11. Feb 18, 2006 #10
    I'd probably say, "Four atoms of Aluminum and six atoms of Oxygen make Aluminum Oxide"...which seems to be quite wrong.
     
  12. Feb 19, 2006 #11

    Gokul43201

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    Try "4 atoms of Aluminum react with 3 molecules of Oxygen, making two molecules of Aluminum Oxide".

    The lesson to learn from this (that's relevant to the question asked) is that 4 atoms of Al give rise to 2 molecules of Al2O3.

    There ! Piece of cake, eh ?
     
  13. Feb 19, 2006 #12
    So you aren't counting the two after the O? If not, why? I was which is how I got confused.
     
  14. Feb 19, 2006 #13
    Determine the number of grams of NH3 produced by the reaction of 3.5g of hydrogen gas with sufficient nitrogen gas. (Check for diatomic elements).

    I wrote down the diatomic elements but I'm not sure if this is right:

    H2 + N2 --> NH3
     
  15. Feb 19, 2006 #14

    Math Is Hard

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    note that he said 3 molecules of Oxygen, not atoms.
     
  16. Feb 19, 2006 #15
    Ah, okay.:biggrin: And that's done with Oxygen and not Aluminum because Oxygen is a diatomic element?:smile:
     
  17. Feb 19, 2006 #16

    Math Is Hard

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    yes, indeedy. Don't forget those diatomic elements --
    Nobody Has Fear Of Ice Cold Beer. :biggrin:
    And you can see there on the left side of the equation that your Al doesn't have a subscript like the O2.
     
  18. Feb 19, 2006 #17
    So, when creating an equation like that one, the diatomic element involved (If there is one.) always has a subscript of two, right? The other element needs no such thing? So, if I'm right about that, my other equation is correct?

    H2 + N2 --> NH3

    Hydrogen and Nitrogen are both diatomic elements so they both require the subscripted two? However, I'm not sure why I ended up with what I did in the first equation...

    Al + O2 --> Al2O3

    The product switched subscripts...Al was three and Oxygen was two at the beginning. Why is that exactly? I remembered doing that with my tutor last year which is why I did it here but why I did it isn't clear to me.
     
  19. Feb 19, 2006 #18

    Math Is Hard

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    yes, but you will need to balance it.
    well, you started with this
    Al + O2 --> Al2O3
    but then you used coefficients to balance it, so your subscripts did not change, just the coefficients.
    4Al + 3O2 --> 2Al2O3
     
    Last edited: Feb 19, 2006
  20. Feb 19, 2006 #19

    Math Is Hard

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    maybe it was the step where you balanced the charges to get Al2O3 that is confusing you?
     
  21. Feb 19, 2006 #20
    I was hoping I had gotten the second one correct (Balancing the equation usually isn't too hard for me but setting it up is a different story.:rofl: ).

    What confused me about the first one is that I started out with this:

    Al + O2 --> Al2O3

    On the left side of the equation, Al has no number near it. Oxygen has a two after it. The final product is Al2O3 which means Al now has a number after it. Where did it come from? I vaguely remember my tutor saying something about, "Switch the numbers..." which is kind of confusing being that it is a vague memory.:rolleyes:

    Now, I know I was given the product in this particular equation but, let's say I was given this:

    Al + O2 --> ?

    ...and I had to write the final product. I'd have to get Al2O3, right? If so, how would I get that all on my own? Am I hitting on something I should know or am I unnecessarily confusing myself?:biggrin:

    As for balancing the second equation, would it be:

    3H2 + N2 --> 2NH3

    How did you get the numbers subscripted in your other post?
     
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