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Finding amount of NaN02 needed to raise the a solution's pH to 5

  1. Feb 18, 2014 #1
    What amount of sodium Nitrate must be added to 500 ml of .200 M solution of HN02. Ka=4.0^-4


    Ka=(H+)(A-)
    --------
    (HA)



    Ph of 5 = 1.0^-5 (H+) so, (1.0^-5)(x)
    ---------- = 4.0^-4 , however X turns out to be way too big
    .200
     
  2. jcsd
  3. Feb 18, 2014 #2

    Borek

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    Staff: Mentor

    Sorry, it is barely unreadable, try to use LaTeX to format the equation.

    If I understand correctly what you did, your approach looks OK. Nothing strange in high amount of nitrate required - you want to get to pH which is 1.6 unit from the pKa, that requires around 101.6 more nitrate than there is nitrous acid.
     
  4. Feb 18, 2014 #3
    Sorry about that, let me try again. Besides the obvious spacing issues I incorrectly mention Sodium Nitrate when it really should be Sodium Nitrite.

    At first the pH of the undisturbed weak acid HN02 is around .69. I found it using a simple ICE table and the initial concentration HN02 (.200 Molar) plus the Ka (4.0^-4). When Sodium Nitrite is added, the salt completely dissociates and you end up with a common ion (NO2-) which would push the equilibrium to the left and result in less (H+) ions and a higher pH. Because we know what the desired pH is (5.00) we know that at equilibrium there should be the Antilog of 5, or 1.0^-5 M of H+. We also know that there has to be a new concentration of NO2- ions which is designated as X. Here is where I get stuck, what would the equilibrium concentration of HN02 be?

    The formula to use would be Ka= (H+)(A-)/(HA)----> 4.0^-4=(1.0^-5)(X)/(equilibrium concentration of HN02?)
     
  5. Feb 18, 2014 #4

    Borek

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    Staff: Mentor

    You can reasonably safely assume equilibrium concentration of HNO2 to be 0.2. High excess of NO2- shifts the dissociation far to the left.

    My approximate calculations show that you need around 8.5M of NaNO2 (that ignoring ionic strength of the solution, which is pretty high).
     
  6. Feb 18, 2014 #5
     
  7. Feb 19, 2014 #6

    Borek

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    Staff: Mentor

    No, you don't have a source of H+ (other than water, but it is an acid many orders of magnitude weaker than HNO2 itself).
     
  8. Feb 19, 2014 #7
    Ah I see, thank you.
     
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