Question

When titrating a weak diprotic acid, pKa2 can be determined as follows:

1. determine the volume of titrant (V2) required to reach the first equivalence point

2. determine the pH corresponding to a volume of titrant equaling 1/2 V2

3. pKa2 = this pH

a. true
b. false

Answer 1

The method described for determining the pKa2 of a weak diprotic acid by measuring the pH at half the volume to the first equivalence point is correct.

Step-by-step explanation:

The statement that pKa2 of a weak diprotic acid can be determined by finding the pH at a volume of titrant equal to half the volume required to reach the first equivalence point (1/2 V2) is true. During a titration, the midpoint occurs when half of the acid has been neutralized by the base. At this point, the concentration of the weak acid ([HA]) equals the concentration of its conjugate base ([A-]), which means the pKa equals the pH based on the equation [H3O+][A-]/[HA]. Since the second ionization constant (Ka2) defines the equilibrium for the second deprotonation process in a diprotic acid, finding the pH at half the volume to the second equivalence point directly gives the pKa2 value.