Which conjugate acid/base pair could be used to create a pH 3.20 buffer solution with the largest possible buffercapacity? a. Sulfurous acid/sodium bisulfite pKa=1.92
b. Hydrofluoric acid/sodium fluoridepKa=3.14
c. Nitrous acid/sodium nitritepKa=3.34
d. Benzoic acid/sodium benzoate pKa=4.20
e. Acetic acid/sodium acetate pKa=4.74

b. Hydrofluoric acid/sodium fluoride (pKa=3.14).

Explanation:

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In this case, since the buffer capacity is related to how good a buffer behaves under the addition of acidic or basic substances in the light of holding the pH as constant as possible, we need to keep in mind that the best buffer must have a pKa as close as possible to the desired pH, that is, if the desired pH here is 3.20, we must pick the acid/base pair which the pKa cosest to 3.20.

In such a way, since b. Hydrofluoric acid/sodium fluoride (pKa=3.14 ) and c. Nitrous acid/sodium nitrite (pKa=3.34) have a pKa close to 3.20, the closest one is b. Hydrofluoric acid/sodium fluoride pKa=3.14 which is 0.06 pH units away from the desired pH, therefore the answer is b. Hydrofluoric acid/sodium fluoride pKa=3.14.

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