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Homework Help: Finding the pH of a buffered System

  1. Apr 3, 2012 #1
    1. A buffer system commonly used in the biochemistry lab uses a substance abbreviated TRIS, and it's conjugate acid TRISH+. A Buffer is prepared with [TRIS] = 0.60M and
    [TRISH+]= 0.30M. What is the pH of this buffered System? The Ka for TRISH+ is 1.20 x 10^-6

    [H+]=Ka [HA]/[A-] (Equation Attached)

    3. I know the answer because I am studying off of a practice exam our professor gave to us, but I'd like to know how to solve it. The method I tried was [(1.20x10^-6)*(0.60/0.30)].

    Thank you in advance!

    Attached Files:

    • 15-1.png
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  2. jcsd
  3. Apr 3, 2012 #2


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    Staff: Mentor

    Equation you listed is just a rearranged version of the dissociation constant. It can be modified even further, yielding so called Henderson-Hasselbalch equation:

    [tex]pH = pK_a + \log \frac {[A^-]}{[HA]}[/tex]

    Once you have it it is just a plug and chug. You just have to understand what is the acid and what is the conjugate base in the case of TRIS.
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