Question

Calculate the volume (in mL of a 0.790 M KOH solution that should be added to 5.500 g of HEPES (MW = 238.306 g/mol, pKa = 7.56) to give a pH of 7.72.

KOH volume = ______ mL

Answer 1

Solution-

Here,

The moles of HEPES = mass/molar mass

moles of HEPES = 5.500 / 238.306

moles of HEPES = 0.023


Now for the acidic buffers

pH = pKa + log [ conjugate base / acid ]

pH = pKa + log [ conjugate base / HEPES]

7.72 = 7.56 + log [ conjugate base / HEPES]


[ conjugate base / HEPES] = 0.16


here total volume is the same


the ratio of conc = ratio of moles

so

moles of conjugate base = 0.16 x moles of HEPES


the reaction is

HEPES + KOH = conjugate base + H20

let us assume the moles of KOH added be y

then moles of the conjugate base formed = y

moles of HEPES reacted = y

moles of HEPES remaining = 0.023 - y


now


y = 0.16 (0.023- y )

y = 0.00368 -0.16y

y= 0.00368/1.16


moles of KOH added = 0.0032

volume of KOH = moles / molarity

volume of KOH = 0.00327 / 0.790

volume of KOH = 0.00414 L


The volume of KOH =4.14 ml


Therefore 4.14 ml of KOH should be added