Question

For an acid-conjugate base pair, when the pH is qual to the pKa, the following is true;

a. The acid concentration is the same as that of the conjugate base.

b. The acid concentration is twice that of the conjugate base.

c. There is only one species present in solution.

d. A and C

e. None of the above

Answer 1

Ans. For a weak acid, the dissociation reaction can written as follow-

                        AH + H₂O --------> A^-(aq) + H⁺(aq)

Using Henderson- Hasselbalch equation-

            pH = pKa + log ([A^-] / [AH])                    - equation 1

            where, [A^-] = base

[AH] = weak acid

Assuming pH = pKa in equation 1-

            pH – pKa = log ([A^-] / [AH])

            Or, 0 = log ([A^-] / [AH])

            Or, [A^-] / [AH] = antilog 0 = 1

            Hence, [A^-] = [AH]

That is, at pH = pKa, the concertation of acid (AH) is equal to the concertation of its conjugate base (A^-).

So,

Correct option is- A.