Question

Calculate the volume (in mL) of a 0.520 M KOH solution that should be added to 6.000 g of HEPES (MW=238.306 g/mol, pKa = 7.56) to give a pH of 7.58.

Answer 1

initial moles of HEPES = mass/(molar mass )
= 6 /(238.306 )
= 0.0252 mol
= 25.2 mmol

let volume of KOH added be V mL
mmol of KOH added = M*V = 0.520*V mmol

0.520*V mmol of HEPES will react with KOH to form 0.520*V mmol of EPES-
mmol of HEPES remaining after reaction = 25.2 - 0.520*V

use:
pH = pKa + log {[EPES-]/[HEPES]}
7.58 = 7.56 + log (0.520*V / (25.2 - 0.520*V))
log (0.520*V / (25.2 - 0.520*V)) = 0.02
0.520*V / (25.2 - 0.520*V) = 1.047
0.520*V = 26.3844 - 0.26175*V
V = 33.75 mL
Answer: 33.75 mL