[Ibin-Huan]

why in buffer solution preparation, the value of pKa should near the pH that we want?

Thank you in advance

[Ibin-Huan]

in buffer mixtures, why salt does not react with water to hydrolyze as an acid or a base?

[Borek]

why in buffer solution preparation, the value of pKa should near the pH that we want?

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

in buffer mixtures, why salt does not react with water to hydrolyze as an acid or a base?

It does, just like acid dissociates, but in most cases both effects are negligible. Sometimes (especially for a very weak or too strong acids) they can't be ignored.