Question

1. Determine the pH of the buffer solution that is prepared by mixing 867 mL of 0.0403 M HEPES−H (C8H18N2O4S ) to 627 mL of 0.120 M HEPES (C8H17N2O4S− ). Assume the 5% approximation is valid. Report your answer to 2 decimal places.

2. Determine the pH of the HEPES−H/HEPES buffer solution after the addition of 21.2 mL of 2.89 M solution of hydrochloric acid to the existing buffer solution. Assume the 5% approximation is valid.

Answer 1

1.
pKa of HEPES-H is 7.5
[HEPES−H ] = number of moles of HEPES−H / total volume
                        = 0.0403*867 / (867 + 627)
                        = 0.0234 M
[HEPES ] = number of moles of HEPES−H / total volume
                        = 0.120*627 / (867 + 627)
                        = 0.0504 M
use:
pH = pKa + log {[HEPES ] / [HEPES−H ]}
       = 7.5 + log {0.0504 / 0.0234}
       = 7.83
Answer: 7.83

2.
when Hcl is added,
HEPES + HCl ----> HEPES-H
number of moles of HCl added = M*V = 21.2*2.89 = 61.268 mmol
So 61.268 mmol of HEPES reacts to form 61.268 mmol of HEPES-H
[HEPES−H ] = number of moles of HEPES−H / total volume
                        = (0.0403*867 + 61.268) / (867 + 627 + 21.2)
                        = 0.0635 M
[HEPES ] = number of moles of HEPES−H / total volume
                        = (0.120*627 - 61.268) / (867 + 627 + 21.2)
                        = 0.0092 M
use:
pH = pKa + log {[HEPES ] / [HEPES−H ]}
       = 7.5 + log {0.0092 / 0.0635 }
       = 6.66
Answer: 6.66