Question

When titrating a weak diprotic acid, pKa1 can be determined as follows:

1. determine the volume of titrant (V1) required to reach the first equivalence point

2. determine the pH corresponding to a volume of titrant equaling 1/2 V1

3. pKa1 = this pH

a. true
b. false

Answer 1

The method of determining pKa1 by measuring the pH at half the volume of titrant to reach the first equivalence point is correct, as the pKa of a weak acid equals the pH at the titration midpoint.

Step-by-step explanation:

The statement that pKa1 can be determined by measuring the pH at half the volume of titrant required to reach the first equivalence point (1/2 V1) is true. At this midpoint in the titration of a weak diprotic acid, the concentration of the acid [HA] is equal to the concentration of its conjugate base [A-]. Consequently, the pKa1 of the weak acid is equal to the pH measured at this midpoint. This is a fundamental principle applied in acid-base titration to determine the pKa values of weak acids.