Prepare a buffer by acid–base reactions. Consider how to prepare a buffer soluti
ID: 949043 • Letter: P
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.
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
Weak Acid Conjugate Base Ka pKaHNO2
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
Explanation / Answer
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