A diprotic acid H₂A has K_a1 = 1 × 10^-4 and K_a2 = 1 × 10^-8. The corresponding base A^2- is titrated with aqueous HCl, both solutions being 0.1 mol L^-1. Which one of the following diagrams best represents the titration curve which will be seen?

Question

Quizplus · Accepted Answer

This is a diprotic acid titration with a strong acid. The pH at the equivalence points depends on the pKa values of the acid. In this case, since the second pKa value is much smaller than the first one, the pH at the second equivalence point will be lower. Therefore, we expect to see two well-defined equivalence points, with the first one at a higher pH and the second one at a lower pH.

Choice A is incorrect because it shows only one equivalence point.

Choice C is incorrect because it shows an equivalence point at a higher pH than the first one, which is not possible.

Choice D is incorrect because it shows an equivalence point at a higher pH than the first one, which is not possible.

Choice E is incorrect because it shows an equivalence point that is too close to the second one.

A Diprotic Acid H2A Has Ka1 = 1 × 10-4

A Diprotic Acid H₂A Has K_a1 = 1 × 10^-4