Question

If you have a list of acids or bases and their conjugate salts, and are asked which pairs create the best buffer, how would you know which are best?

Answer 1

The salt contains the conjugate base or acid of the corresponding acid or base on the list. Since most buffers are combinations of a weak acid and its conjugate base, or a weak base and its conjugate acid, the combination of the salt and the related acid or base make a buffer mixture. That is because one of the ions in the salt is conjugate to one of the substances on the list. Examples:
1) NaC2H3O2(salt) with HC2H3O2(acid) = HC2H3O2 / C2H3O2-
and
2) NH3(base) with NH4Cl(salt) = NH3 / NH4+

Answer 2

Usually the "best" buffer means one which is buffered at a particular pH. Look at the pKa of the acid or base and see if that pKa is near where you want to buffer. If so that is a good buffer for you. For example, the pKa for acetic acid is about 4.74. Therefore, acetic acid/sodium acetate will buffer in that range.