With regards to my last question about the acid. Just to clarify, the [HA] that I should use is 0.12 which is the concentration of acid-concentration os base?

Question

right. (HA) = 0.12 [that's 0.20 - 0.08 = 0.12). And the base, which is (A^-), is 0.08, or course.

Answer 1

To clarify, the concentration of (HA) is 0.12, which represents the acid in the solution. The concentration of the base, (A^-), is 0.08. (HA) is the acid, and (A^-) is the conjugate base.