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 basic buffer solution

When [salt] = [base]

pH = pKa

But pKa = 8.3

Hence, pH = 8.3

Hence, at pH 8.3 you would expect Tris to act as a buffer in a solution that has equal molar concentrations of base and its conjugated acid.

b)

100 ml of buffer with total concentration (base and conjugate acid) of 300 mM corresponds to

Out of this, 15 mmol corresponds to base and 15 mmol corresponds to salt (because the solution has equal molar concentrations of base and its conjugated acid).

After addition of 4.0 mmol NaOH, the Tris base concentration will increase and salt concentration will decrease as NaOH will react with salt to give Tris base.

c)

After addition of 6.0 mmol HNO3, the Tris base concentration will decrease and salt concentration will increase as HNO3 will react with Tris base to give salt.