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Homework Help: Write an equation to illustrate that a reaction is acidic.

  1. Dec 29, 2014 #1
    1. The problem statement, all variables and given/known data

    Write equations to illustrate whether P4O10(s) is an Arrhenius acid or a base.

    2. Relevant equations

    P4O10(s) + H2O(l) will eventually have the H3O+ and a negative ion because it is a non-metallic oxide in water.

    3. The attempt at a solution

    I can think of at least 10 different ways to come up with a H3O+ in the equation, but the answer has a specific answer which is,

    P4O10(s) + 6H2O → 4H3PO4(aq)

    I don't know why they have 6 H2O's.

    I also don't know why they only use one of the 4 H3PO4(aq) for this equation,

    H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4-(aq) .

    I can't find anywhere in the chapter that explains how I would know this.
  2. jcsd
  3. Dec 29, 2014 #2


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    Gold Member

    Try balancing the equation.
    Did you wish to count individual molecules, or the stoichiometry?
  4. Dec 29, 2014 #3


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    For the first question you might find it easier to write it as P2O5, write an obvious covalent structure for it with pentavalent P. See how H2O can be added to that in two stages.

    For your second question, the means to answer it may be in another chapter - on buffers, pH calculations etc. All three protons of H3PO4 can diissociate. But only the first one is strongly acid. Actually what they write seems to me slightly arbitrary. From the first Ka of around 0.01 M it follows (easily :biggrin:) that if you dissolved the pentoxide to around 0.01 M [P] about half of it would be in form H2PO4-; the greater the dilution the greater this fraction, while at higher concentrations the majority would be undissociated H3PO4 - e.g. at 1M about 90% is, and only 10% is H2PO4-, and very little other ionised forms.

    Rereading maybe that was not your question. The scheme is not meant to represent 'only one' ot the four molecules - it represents any of them equally. That reaction is extremely fast and goes back and forth something like 108 times per second.
    Last edited: Dec 30, 2014
  5. Dec 29, 2014 #4
    I understand that the equation is balanced. I just have no idea why they decided to end up with 4H3PO4(aq) instead of anything else. Of all of the possible molecules, how would I know to try using that molecule?

    This is half way through grade 11 chemistry (old student who is upgrading). I think stoichiometry is in the next chapter. So I think individual molecules would be expected for this question.
  6. Dec 29, 2014 #5


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    Staff: Mentor

    First - P4O10 is an acid anhydride and is known to react with water producing phosphoric acid. My bet is you were told that earlier.

    Second - phosphoric acid is an obvious example of an acid containing P, and being able of producing H3O+.
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