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Find which nitrate has the lowest pH

  1. Jun 29, 2013 #1
    1. The problem statement, all variables and given/known data

    Which solution will have the lowest pH?
    a) Mg(NO3)2
    b) Ca(NO3)2
    c) Al(NO3)3
    d) KNO3
    e) Zn(NO3)2

    2. Relevant equations

    None I know of.

    3. The attempt at a solution

    My reasoning was that when the metal dissociates from the nitrate, it will leave a negative charge on the nitrate. I need to find which one will have the lowest pH, so I figured that the KNO3 would dissociate and leave the NO3 with 1 negative charge, which would only attract 1 hydrogen ion, leaving the least amount of OH ions, meaning the solution would have a lower pH than the other ones, since they would dissociate and attract more hydrogens, leaving more OH ions, which would cause the pH to be higher.
    But apparently that's not the case.

    So I'm stumped. Please help.
     
  2. jcsd
  3. Jun 30, 2013 #2

    Borek

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    Staff: Mentor

    Men+ + mH2O <=> Me(OH)m(n-m)+ + mH+
     
  4. Jun 30, 2013 #3
    I don't understand.
     
  5. Jun 30, 2013 #4
    Borek wanted to convey that the metal ions would form hydroxides through reaction with water.

    The metal ion which would use more OH- will result in increased concentration of H+. (H2O <=> OH-+H+)
     
  6. Jun 30, 2013 #5
    See which one gives,more
    H3O+
    H+

    Lowest pH means acids
     
  7. Jun 30, 2013 #6
    I learned weeks ago, that you can find it by oxidation number. If I remember clearly below number the metal has, it's acid
     
  8. Jun 30, 2013 #7
    Is the the answer KNO3?
     
  9. Jun 30, 2013 #8

    Borek

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    Staff: Mentor

    No.

    No.
     
  10. Jun 30, 2013 #9
    As usually I suck at chemistry. Could u explain more easy way to determine it? Sorry for my English
     
  11. Jun 30, 2013 #10

    Borek

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    Staff: Mentor

  12. Jun 30, 2013 #11
    Oh, I was confused what Me and m were for. I guess m was just used as a variable and Me stands for metal.

    So the opposite of what I thought was true. When the Al(NO3)3 dissociates, that leaves an Al with a +3 charge, which will bond to 3 OH, which will leave 3 H cations. For the one I chose, K will only bond to 1 OH, leaving just 1 H cation, which will have a higher pH. So the lowest pH will be the Al(NO3)3.

    Thanks guys.
     
  13. Jun 30, 2013 #12

    Borek

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    Staff: Mentor

    Yes, Me is a metal, and m, n are just some integeres.

    Beware - these are all equilibrium reactions. It doesn't mean these reactions proceed to an end, so your conclusion is not necessarily correct. TBH without consulting solubility/stability constant tables I am not sure what is the correct answer here.
     
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