Question

Prepare a buffer by acid–base reactions.

Consider how to prepare a buffer solution with pH = 9.31 (using one of the weak acid/conjugate base systems shown here) by combining 1.00 L of a 0.316-M solution of weak acid with 0.351 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	9.40

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

L

Answer 1

moles of weak acid = 0.316 x 1 = 0.316

moles of KOH = 0.351 x V = 0.351 V

the best combination HCN + CN- . because its pKa value nearer to pH value

pKa = 9.40

pH = 9.31

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

0.316 0.351V 0 0 -------------------> initial

0.316-0.351V 0 0.351V 0.351V ----------------> after reaction

pH = pKa + log [KCN/HCN]

9.31 = 9.40 + log (0.351 V / 0.316-0.351V)

0.813 =0.351 V / 0.316-0.351V

V = 0.404 L

volume of KOH needed = 0.404 L