Question

A weak acid, HA, has a Ka of 1.4 X 10−3. In order to produce a buffer solution with a pH of 2.25, the ratio of [A− ]/[HA] must equal:

A) 4.0

B) 2.0

C) 1.0

D) 0.25

E) none of the above

Answer 1

Ans. Henderson- Hasselbalch equation, pH = pKa + log ([A^-] / [HA])

Where, [A^-] = concentration of the conjugate base

            [HA] = concentration of acid

Given, Ka = 1.4 x 10^-3

            pKa = - log Ka = - log (1.4 x 10^-3) = 2.85

            Thus, pKa = 2.85

Putting the values in HH equation-

            2.25 = 2.85 + log ([A^-] / [HA])

            Or, 2.25 - 2.85 = log ([A^-] / [HA])

            Or, ([A^-] / [HA]) = antilog (0.6) = 0.25

Thus, [A^-] / [HA] = 0.25

Correct option. D. 0.25