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Homework Help: Henderson Hassalbach equation problem

  1. Apr 17, 2010 #1
    1. The problem statement, all variables and given/known data
    I am trying to prepare a 1L Tris-HCl buffer 1M pH7. The pKa value is 8.06. The question is, how many molar of HCl should be added to produce this buffer?


    2. Relevant equations
    pH = pKa + log [base/acid]


    3. The attempt at a solution
    7=8.06 + lg (a/b)
    -1.06=lg (a/b)
    a/b = 0.0871 -----1
    a+b= 1M
    a=1-b --------2
    Hence,
    (1-b)/b = 0.0871
    b = 0.92M
    a = 0.08M
    So, the molarity of HCl to be added is 0.92M or 0.08M. Why?
     
  2. jcsd
  3. Apr 17, 2010 #2

    Borek

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    Staff: Mentor

    What reaction takes place when you add HCl?

    --
     
  4. Apr 17, 2010 #3
  5. Apr 18, 2010 #4

    Borek

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    Staff: Mentor

    No. HCl is neutralized, but there is no water between products. TRIS hydrochloride is produced.

    Buffer is a mixture of salt & acid/base, but acid and conjugate base.

    You need to identify an acid and its conjugate base in the case of TRIS.

    What is a Broensted acid? What is a Broensted base? How are they related?

    --
    methods
     
  6. Apr 18, 2010 #5
    So buffer should be acid+conjugate base or base + conjugate acid.
    Bronsted Acid donate proton while base receive proton.
    HCl is suppose to be the acid cuz it has H+ to be donated.

    If no water product, could it be
    (CH2OH)3CNH2 + HCl <> (CH2OH)3CCl + NH3?
    but still it doesn't become TRIS hydrochloride..
    Or it becomes (CH2OH)2CClNH2+ CH3OH? I can't hink of other possiblity..
     
    Last edited: Apr 18, 2010
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