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Homework Help: Biochemistry help

  1. Oct 1, 2009 #1
    1. The problem statement, all variables and given/known data
    Hey, have this question which i just cant wrap my head around, any help would be appreciated.

    Formic Acid (HCOOH) - pKa = 3.8 is a weak acid that partially dissociates to form its conjugate base (HCOO-) and releases protons into aqueous solutions. Whats the pH of a solution in which (HCOOH) = .7M and (HCOO-) = .15M?

    Then, if the pH of sol. of Formic acid has been titrated to a final pH of 2.8, Whats the ratio of conjugate base : Weak acid?

    And suppose your given stock solutions of formic acid and sodium salt of the conjugate base : (HCOOH) = 3.OM and (HCOONa) = 1.5M,
    How would you make a .1 M buffer(total formic acid + sodium formate concentration), 100mL total at pH 3.5, using above solutions?

    2. Relevant equations

    3. The attempt at a solutionHasselback equation
  2. jcsd
  3. Oct 1, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Maybe this will start the neurons firing....

    If you consider the equilibrium of formic acid to be

    HCOOH <-----> H+ + HCOO-

    and you know the concentration of HCOO-, how much H+ will you necessarily have?

    Yes, write out the Henderson Hasselbalch equation and see if you notice anything....

    pH = pKa + Log([A-]/[HA])
  4. Oct 1, 2009 #3
    ok got 2.3 as pH, is that right? Still not sure bout ratio. got 4.1 for the first part. Please help on the last part!? havent clue what to do with buffer
  5. Oct 2, 2009 #4


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    Staff: Mentor

    2.3 - show how you get there.

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