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Chemistry: How do I write an ionic equation for this reaction?

  1. Jun 14, 2012 #1
    1. The problem statement, all variables and given/known data

    I was doing this problem for HW online, that asks to write the ionic equation for a reaction, but I'm completely stuck. After using all my attempts (i.e., I can no longer submit the answer even if it is right, :frown: ) I still couldn't get the right answer. We've only done very simple ionic equations in class, so I couldn't apply what we did in class to the problem. Anyway here is the reaction to make an ionic equation for:

    [tex]Ba(OH)_2 (aq) + H_{3}PO_{4} (aq) \rightarrow [/tex]

    2. Relevant equations

    Not applicable.

    3. The attempt at a solution

    Here is the last answer I attempted. Wolframalpha told me that the compound phosphoric acid (H3PO4) was "very soluable" so I separated it into ions. That was about all it was able to help me with. I assumed that the phosphate separated from the hydrogen atoms to combine with the barium and create an insoluable solid (according to my textbook, any non-alkali metal combined with phosphate is insoluable). So with all that, I thought the ionic equation would logically be:

    [tex]3H^{+}(aq) + 2{PO_4}^{-3}(aq) + 3Ba^{+2}(aq) + 2OH^{-}(aq) \rightarrow Ba_3(PO_4)_2(s) + 3H^{+}(aq) + 2OH^{-}(aq)[/tex]

    This is wrong as my fifth and final attempt told me . . . The super vague reason the system gives me for this being wrong is that "Your answer contains an ambiguous or incomplete reaction equation. Check all the components on the reactant-side of the equation. Check all the components on the product-side of the equation." If you need anymore information on my thought process or anything like that, please ask. Thanks in advance for any help.
     
  2. jcsd
  3. Jun 14, 2012 #2
    Do you need to remove the spectator ions?
     
  4. Jun 14, 2012 #3
    For the net ionic equation, yes, I have to remove the spectator ions. But for this equation (just the plain old ionic equation) I must keep the spectator ions.
     
  5. Jun 14, 2012 #4

    Borek

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    Staff: Mentor

    H++OH- - don't you think they will react?

    Phosphoric acid is soluble, but it is a weak acid, so it is not dissociated completely.

    Barium hydroxide is weakly soluble, but it is a strong base, so it is completely dissociated.

    It is a little bit tricky if you ask me. Not clear to me what they can expect.
     
  6. Jun 14, 2012 #5
    Umm, do they make water? I'm not sure.

    How do you represent something that only partially dissociates in the equation, though?
     
  7. Jun 14, 2012 #6

    Borek

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    Staff: Mentor

    Yes.

    That's why I have no idea what kind of answer they expect. 1M solution of phosphoric acid acid is approximately 8% (H++H2PO4-) and 92% H3PO4. It is not something you can write in a simple way.
     
  8. Jun 14, 2012 #7
    Wow we definitely didn't learn that in class. I think based on other problems we're supposed to make the simplifying assumption that the compounds either completely separate or don't separate at all. Is there any way to do this problem with that in mind?
     
  9. Jun 15, 2012 #8

    Borek

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    Staff: Mentor

    The closest would be to assume molecular (undissociated) H3PO4.
     
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