[coolusername]

Tris(hydroxymethyl)aminomethane [(HOCH2)3CNH2, commonly known as "Tris"] is very widely used as a buffer in biochemical and molecular biological work. The pKa of
Tris is 8.1. A buffer was prepared by dissolving 100 mmols of Tris in about 900 ml of water,
adjusting the pH to 8.5 by addition of HCl (using a pH meter to monitor pH) and finally
adjusting the final volume to 1 liter. This is called a Tris-HCl buffer.
a) Is Tris an acid or a base?
b) How should the buffer solution be labelled?
c) What is the ratio of [Tris]/[Tris- H+] in the solution?
d) Assuming the pH meter was properly calibrated, what fraction of the Tris is protonated (i.e.
in the Tris-H+ form)?
e) What is the concentration of Cl- in the solution?

[billnotgatez]

forum rules require you show some work
please read forum rules

[Babcock_Hall]

You obviously know how to use the H-H equation.  That should help.

[coolusername]

What I did was:

a) Tris is a base. (HCl is added to the solution of Tris thus it must be the conjugate base of Tris-H)

b) I'm not sure about this question but I labelled it as Tris-HCl (0.11M Tris Base, pH 8.5)

c) 8.5 = 8.1 + log([A-]/[HA])
   0.4 = log([A-]/[HA])
   10^0.4 = [A-]/[HA]
   2.512/1 = [A-]/[HA]

d) 1/(1+2.512) = 1/3.512 ≈ 2/7

e) I know that HCl is a strong acid thus it would dissociate completely. However I don't have the concentration of HCl.

I'm pretty sure that I did a) c) and d) correctly but I'm not sure about b) and e).


Any feedback would be very helpful!

[Babcock_Hall]

Any chloride ion you added in any form is not going away.  When I label buffers, I use the concentration that is the sum of the acid and base forms of the buffering species.