Question

if you make up a solution of 100mL of 0.1M HEPES in the basic form, what will be the pH?

Answer 1

HEPES is a weak acid. The pKa value of this acid is 7.5. The pKb of HEPES can be calculated as given below

pKa + pK_b = 14

pK_b = 14- pK_a = 14 - 7.5

pK_b =14 - 7.5

= 6.5

Now calculate the K_a value of HEPES as

K_b = -log(pK_b)

= -log(6.5)

= 3.16 x 10^-7

As the solution is prepared in basic condition having concentration of 0.1 M. Thus, the [OH^-] of weak base can be calculated as

[OH^-] = (K_b x c)^1/2

= [(3.16 x 10^-7)(0.1M)]^1/2

= 1.78 x 10^-4M

Using above value, calculate the pOH value as

pOH = -log(OH^-)

= -log(1.78 x 10^-4M)

= 3.75

Now calculate the pH of solution as

pH = 14 - pOH

pH = 14 - 3.75

pH = 10.25

Thus, the pH of the solution will be 10.25.