What is the optimum pH of a sodium formate/formic acid buffer? (For formic acid, K_a = 1.7 * 10^-4.)

Question

Quizplus · Accepted Answer

To find the optimum pH of a buffer solution, we use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

Where pKa is the dissociation constant of the acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.

Given the formula of formic acid, we can calculate its pKa:

pKa = 1/2 (pKs + pKb - log(Kw))

Where pKs is the dissociation constant of the acid and pKb is the dissociation constant of the conjugate base. Kw is the ionization constant of water.

pKs = 1.7
pKb = 14 - pKs = 12.3
Kw = 1.0E-14

pKa = 1/2 (1.7 + 12.3 - log(1.0E-14)) = 3.77

Therefore, the optimum pH is when pH = pKa = 3.77. The answer is B.

What Is the Optimum PH of a Sodium Formate/formic Acid