Question

at the half - equivalence point of the first dissociation step in a diprotic acid titration, the ph is equal to...
...the initial ph of the acid solution.
...pka1.
...pka2.
...7.00.

Explanation:

Step1: Apply Henderson - Hasselbalch equation

For a diprotic acid \(H_2A\), in the first dissociation step \(H_2A
ightleftharpoons H^++HA^-\). At the half - equivalence point of the first dissociation step, \([HA^-]=[H_2A]\).
The Henderson - Hasselbalch equation is \(pH = pKa+\log\frac{[A^-]}{[HA]}\). Here, for the first dissociation, when \([HA^-]=[H_2A]\), \(\log\frac{[HA^-]}{[H_2A]} = 0\). So \(pH=pKa_1\).

Answer:

...pKa1.