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Homework Help: Find total ionic (balanced equation) for this particular reaction.

  1. Oct 21, 2012 #1
    1. The problem statement, all variables and given/known data

    Find the total ionic (balanced) equation of the reaction of calcium hydroxide and phosphoric acid in aqueous solution.

    2. Relevant equations

    Solubility rules

    3. The attempt at a solution

    First, I balanced the reaction.
    3Ca(OH)2 + 2H3PO4 --> Ca3(PO4)2 + 6H2O
    The part I'm stuck at is I dont know what to break up and what to leave. I know the solubility rules, but for some reason I keep getting this question wrong.

    I know that Ca(OH)2 can break up into 3Ca2+ + 6OH-
    But is that the only one that breaks up?
    I looked it up and I'm getting mixed results. Please help me if you can. Thanks in advance.
  2. jcsd
  3. Oct 21, 2012 #2


    User Avatar

    Staff: Mentor

    No idea what the correct answer is, as unfortunately it depends on what the teacher had on mind :frown:

    Calcium hydroxide is weakly soluble (so mostly present as a solid), but it is a strong base (so it is 100% dissociated).

    Phosphoric acid is a weak acid, so in typical solution it will be dissociated only partially, and only the first proton will be dissociated in substantial amounts (for example in 1M solution about 8%), yet it can react all the way down to PO43-.

    Apparently you are expected to ignore some of these information and give answer based on some general (but not always correct) rules.
  4. Oct 21, 2012 #3
    Thank you so much. I figured it out. They wanted me to look as phosphoric acid and calcium phosphate, both as insoluble. Thanks again!
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