Question

The weak base commonly known as Tris [ tris(hydroxymethyl)aminomethane], has pKa = 8.3 at 20^oC and is commonly used in biochemical buffer solutions.

a) At what pH would you expect Tris to act as a buffer in a solution that has equal molar concentrations of base and its conjugated acid?

b) What is the pH after addition of 4.0 mmol NaOH to 100 ml of buffer with pH found in part a) and total concentration (base and conjugate acid) of 300 mM?

c) What is the pH after addition of 6.0 mmol HNO3 to the same buffer as in part b)?

Answer 1

a)

for a buffer equation apply Henderson Haselbalch equation

pH = pKa + log(A-/HA)

for a conjguate= acid solution

then

log(1) = 0

best then

pH = pKa = 8.3

th eoptimal pH will be 8.3 for this buffer

b)

pH after adding:

NaOOH

mmol of base = MV = 100*4 = 4 mmol

mmol of TRIS = 30 mmol

so..

4 mmol of conjguate forms, 30-4 = 26 mmol of TRIS left

pH = 8.3 + log(26/4) = 9.112

c)

mmol of acid =6

so

conjguate = 30-6 = 24

reacted acid = 6

pH = 8.3 + log(6/24) = 7.69794