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Concentration Problem

  1. May 31, 2013 #1

    I have a problem on my hands, I'm trying to find the concentration of ammonium phosphate in mol/L.


    Excuse the badly written equation, I'm not quite sure on how to use subscrips/arrow signs on here.

    (NH4)3P04 --------------- 3NH4 + P04

    Known Data:

    Mass of 3NH4 measured with probe = 400 mg/L (0.40 g)
    Mass of P04 measured with probe = 10 mg/L (0.01 g)

    Moles of 3NH4 = 0.0678 moles
    Moles of P04 = 0.00009915 moles

    The problem:

    I have found the mass of each of the products using a probe. The 3:1 ratio is not existent and this is due to a number of reasons (ignore this). How do I determine the mass of the original substance put in. Once I know that I can find the morality quite easily. I've attempted it but my high school teacher said I was wrong. I added the masses and now when I look back, it was a stupid thing to do. I think I need to use the solubility of ammonium phosphate some how, the solubility in water is 58g/100mL

    Any help would be much appreciated, I really am stuck.
    Last edited: May 31, 2013
  2. jcsd
  3. May 31, 2013 #2


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    Staff: Mentor

    If concentrations don't reflect the original composition, there are reactions that consumed some of the original substance. If so, in general you can't tell what was its initial concentration. Could be there are additional properties of your system that can be used to find the answer, but you haven't said anything about them.
  4. May 31, 2013 #3
    Thanks for the reply. Yeah I understand what you are saying, my teacher told me to look into Le Chatelier's principle. As far as other properties go, I'm not quite sure what you mean. I have obtained other results through testing that might be of benefit. Such as the pH levels, TS (total solids) and TDS (total dissolved solids). Although, I'm not quite sure if that's what you were after. I'll keep researching though.
  5. May 31, 2013 #4


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    Staff: Mentor

    What kind of probe have you used, what are the reasons that you know about that made the ratio between ammonia and phosphate differ from 3:1?
  6. May 31, 2013 #5
    To measure ammonium, an ammonium ion selective electrode was used. For phosphate, a colorimeter was used. As far the ratio, I'm assuming that there is something (some element in the water sample) that is causing the phosphate to be smaller than it should be. At least that is what I think. My teacher told me to look into what decreases the value for phosphate, but I haven't found anything yet.
  7. May 31, 2013 #6


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    Staff: Mentor

    Perhaps it was precipitated by Mg or Ca, or complexed some other cation (iron) that was present in water. Total concentration of ammonia is a function of both ammonium ion concentration and pH (although if you have pH below 8 it shouldn't matter much).
  8. May 31, 2013 #7
    I'll research that up. Thanks for the help, I think I might know what to do now :).
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