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How to determine strong acids when reacting with magnesium?

  1. Mar 28, 2012 #1
    1. The problem statement, all variables and given/known data
    I have to design a lab which will determine nitric, hydrochloric and sulfuric acid from containers A, B and C - i.e. It is unknown which acid is held in which container. This has to be done by reacting the unknown acids with magnesium


    2. Relevant equations
    Mg + 2H(+) → Mg(2+) + H2


    3. The attempt at a solution
    So sulfuric acid is a diprotic acid and therefore once it has dissociated its first proton [becoming HSO4(-)] which we can assume happens at 100% effectiveness, the hydrogen sulfate will further dissociate into SO4(2-) but this is a weak acid and therefore only partially dissociates its H(+) ion. This means that my rate of hydrogen gas production will be higher than the other two strong acids (if only marginally) due to a higher concentration of H(+) ions. I assume that hydrogen gas production rate would be my source of determining the acid??

    Then comes the problem: how would I be able to see a noticeable difference in nitric acid and hydrochloric acid hydrogen gas production rates? Both are strong, monoprotic acids and therefore we can assume 100% dissociation of their protons. Would we have to refer to acid dissociation constants and therefore be able to say 'x' had a faster rate of hydrogen gas production than 'y' and therefore this means that 'x' is HCl as this has a higher K(a) value and thus will have a greater concentration of H(+) ions at equilibrium than 'y' leading to a greater production of hydrogen gas according to the 'relevant formula'.

    Would really appreciate any help that can be contributed. Oh, and I'm new here so please correct me on any little formalities that I should have done/abided to.
     
  2. jcsd
  3. Mar 29, 2012 #2

    Borek

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    Staff: Mentor

    You have not stated if you are given concentrations of these acid. If not - you can't do much.

    Nitric acid is relatively easily reduced by evolving hydrogen to ammonia.
     
  4. Mar 29, 2012 #3
    Apologies Borek. I DID forget to mention that we know the concentration of all the acids are 1 molar solutions.
     
  5. Mar 29, 2012 #4

    Borek

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    Staff: Mentor

    No problem.

    If so, combine the information about amount of hydrogen produced and consumed in the reaction mixture. Ammonia dissolves in water pretty well. I guess that should be enough to determine which acid is which.
     
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