Question

Can you tell if I'm correct. Which combination in titration has ph=pka and buffer region

strong acid-strong base- no to both
strong acid-weak base- yes to both
weak acid-strong base- yes to both
weak acid-weak base- I'm not sure about this one. Can anyone explain this combination

Answer 1

Correct on 1.
Correct on 2.
Correct on 3. To be a buffer, the solution must be a weak acid and its salt (conjugate base) OR a weak base and its salt (conjugate acid). Thus, adding a strong base, such as NaOH, to a weak acid, such as acetic acid, forms sodium acetate and you leave some acetic acid un-neutralized. (An xample of a weak base and its salt is aqueous NH3 and NH4Cl.)

A weak acid/weak base combination certainly has a pKa (both the weak acid AND weak base will hav a pKa). Strictly speaking, this combination does not follow the definition I gave above, however, certain weak acid/weak base combinations provide limited buffering in certain regions. For example, ammonium acetate, a salt of a weak acid and weak base, provides buffering in the 7.0 pH region.