Question

A) If the weak acid in a buffer system has pKa 10.5, in what pH range is the system a most effective buffer?

B) In the phosphate buffer system containing K2HPO4 and KH2PO4, what is the weak acid? What is its conjugate base?

Answer 1

A) It is generally assumed that a buffer is most effective if the pH of the buffer is within the range (pKa-1, pKa+1). So we can say the buffer is most effective in the desired pH range of (9.5,11.5). However it should be noted that the closer is the desired pH to the pKa of the weak acid, the better buffer action the solution offers.

B) H2PO4 -    HPO4 2- + H+

So clearly as a proton is released from KH2PO4 as in an acid and K2HPO4 gains the proton. Thus KH2PO4 is the weak acid and K2HPO4 is the conjugate base.