Question

Consider how to prepare a buffer solution with pH = 9.52 (using one of the weak acid/conjugate base systems shown here) by combining 1.00 L of a 0.417-M solution of weak acid with 0.284 M potassium hydroxide.

Weak Acid	Conjugate Base	Ka	pKa
HNO2	NO2-	4.5 x 10-4	3.35
HClO	ClO-	3.5 x 10-8	7.46
HCN	CN-	4.0 x 10-10	9.

How many L of the potassium hydroxide solution would have to be added to the acid solution of your choice?

?L

Answer 1

Given that

pH = 9.52

Concentration of KOH = 0.284 M

moles of weak acid = 0.417 x 1

= 0.417

moles of KOH = 0.284 x V

= 0.284 V

As its pKa value nearer to pH value , the best combination is HCN + CN- .

pKa = 9

Let us consider a reaction

HCN + KOH -----------> KCN + H2O

0.417 0.284V 0 0

0.417-0.284V 0 0.284V 0.284V

We know,

pH = pKa + log [KCN/HCN]

9.52 = 9 + log (0.284V) / (0.417-0.284V)

log (0.284V) / (0.417-0.284V) = 3.31

V = 1.13 L

volume of KOH needed = 0.404 L