Finding the pH of a buffered System

1. Apr 3, 2012

Glitche

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)].

File size:
887 bytes
Views:
52
2. Apr 3, 2012

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:

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

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.