3 a b c One liter of a 1M Tris buffer pKa83 was prepared and

3) a. b. c. One liter of a .1M Tris buffer (pKa-8.3) was prepared and adjusted to a pH of 2.0. What are the concentrations of the conjugate base and weak acid at this pH? Is this an effective buffer? Explain What would be the percent of protonation of Tris at pH2?

Solution

Pka of Tris base = 8.3

pH = 14 - (pkb + log(Tris-HCl/Tris base)) (pka = 14-pkb)

pH = pka - log(Tris-HCl/Tris base))

total buffer = V*M = 1*0.1 = 0.1 mol

   Tris-HCl+Tris base = buffer

2 = 8.3-log(x/(0.1-x))

x = 0.099

No of mol of Tris-HCl = x = 0.099 mol

concentration of weak acid = Tris-HCl = 0.099/1 = 0.099 M

concentration of conjugate base = (0.1 - 0.099)/1 = 0.001 M

b. it is not effective buffer,

effective buffer range : pka+1 to pka-1 or 9.3 to 8.3.

c. at pH2 = 0.099/0.1*100 = 99%