Question

How are the pK’s of the conjugate acid and base are related to each other?

Answer 1

We know that $pK_{a}$ = log of acid constant and

                             $pK_{b}$ = log of base constant

They both are related only when the acids and bases are conjugate to each other .

For example – i) CH$_{3}$COOH and CH$_{3}$COO$^{-}$  

                        ii) H$_{2}$PO$_{4}^{-}$ and HPO$_{4}^{2-}$

Generally,  $pK_{a}$ +$pK_{b}$  = $pK_{w}$

Here $pK_{w}$  is the water constant.

Only for  $pK_{a}$  and $pK_{b}$  that describe the same reaction  is it true that $pK_{a}$ +$pK_{b}$ = 14

Answer 2

We know that pK_{a}pKa​ = log of acid constant and

                             pK_{b}pKb​ = log of base constant

They both are related only when the acids and bases are conjugate to each other .

For example – i) CH_{3}3​ COOH and CH_{3}3​COO^{-}−  

                        ii) H_{2}2​ PO_{4}^{-}4−​ and HPO_{4}^{2-}42−​

Generally,  pK_{a}pKa​ +pK_{b}pKb​  = pK_{w}pKw​

Here pK_{w}pKw​  is the water constant.

Only for  pK_{a}pKa​  and pK_{b}pKb​  that describe the same reaction  is it true that pK_{a}pKa​ +pK_{b}pKb​ = 14