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How to make a sodium carbonate/sodium hydrogen carbonate buffer

  1. Apr 24, 2012 #1
    1. The problem statement, all variables and given/known data
    "Plan how to make 100mL of a buffer solution with a pH of 10.8 to be made using only sodium carbonate, sodium hydrogen carbonate and water"

    2. Relevant equations
    pH= pKa+ log(CO32-)/(HCO3-)


    3. The attempt at a solution
    I tried rearranging it, but I'm not sure what to do to find both of the concentrations...

    thanks :)
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 24, 2012 #2

    Borek

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    Staff: Mentor

    Calculate ratio of concentrations of carbonate and hydrogencarbonate, see how you can obtain this ratio mixing given reagents.
     
  4. Apr 24, 2012 #3
    ok, I did that and I got the ratio to be 3.55. I then tried to find the conc. of H+, therefore the conc. of carbonate ions... is this how you're supposed to do it?

    it also says "you should specify the amounts of sodium carbonate and sodium hydrogen carbonate that you would use"


    thanks
     
  5. Apr 25, 2012 #4

    Borek

    User Avatar

    Staff: Mentor

    No, once you know ratio of carbonate and hydrogencarbonate it is just a trivial calculation of concentrations - ignore dissociation and equilibrium, just assume you are preparing some volume of the buffer, you want it to be - say - 0.01M in carbonate, use the ratio you already know to calculate concentration of hydrogencarbonate, then calculate how much of each solid you need.
     
  6. Apr 25, 2012 #5
    ok :) thanks! it was a lot simpler than I imagined it to be...
     
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