# Are pKa values of acid and conjugate base the same?

When you look at $$\mathrm{p}K_\mathrm{a}$$ table, you can read $$\mathrm{p}K_\mathrm{a}$$ values of acids. Do conjugate bases also have $$\mathrm{p}K_\mathrm{a}$$ values?

If both can have $$\mathrm{p}K_\mathrm{a}$$ values, why $$\mathrm{p}K_\mathrm{a}$$ value of conjugated bases are same as acids? Can acid have only $$\mathrm{p}K_\mathrm{a}$$ values, not $$\mathrm{p}K_\mathrm{b}$$ values?

Let consider the case of the phosphoric acid.

The phosphoric acid $$\ce{H3PO4}$$ has $$pK_\mathrm{a1}$$.
Its conjugate base $$\ce{H2PO4-}$$ has $$pK_\mathrm{b1} = 14- pK_\mathrm{a1}$$.

But as $$\ce{H2PO4-}$$ is at the same time a ( weaker ) acid as well,
it has its own acidity constant $$pK_\mathrm{a2}$$ independent on $$pK_\mathrm{a1}$$ and $$pK_\mathrm{b1}$$.

Generally, acids like $$\ce{HA-}$$ have $$pK_\mathrm{a2}$$ and $$pK_\mathrm{b1}$$ values.
The former associated with $$\ce{HA- <=> H+ + A^2-}$$,
the latter associated with $$\ce{HA- + H2O <=> H2A + OH-}$$

• thank u for answering my question, but i have a few more questions. 1. pKa2 values exist only weak acid and base, am I right? ex) HCl(strong acid) has no second ionization like H2PO4. 2. I got that a lot of substance have pKa, pKb values both. but when you look at pKa table of Your book, like organic chemistry. then Acids and conjugate bases are same values and honestly, i don't know why they take same values. why hydroxide ion's value is 15.74? is there any relation with mole concentration? – a wander around chemistry Mar 25 at 13:04
• About 1., it is not true. $\ce{HSO4-}$ or $\ce{(COOH)_2^-}$ are still strong acids. About 2., If an acid dissociates only once, both the acid and conjugate base may refer in the table to the same reaction, equilibrium and constant. It would help, if you provide a particular quatation of such a source. – Poutnik Mar 25 at 13:16
• Here the link is. masterorganicchemistry.com/2010/09/29/how-to-use-a-pka-table Is there any particular classfication of determining strong or weak acid? Once I learned from book, strength is relative which substance dissociate in water – a wander around chemistry Mar 25 at 13:20
• And what is the problem there ? The mentioned $\mathrm{p}K_\mathrm{a}$ always relates to the acid side of the pair, not matter if we do not consider it normally as an acide, like methane. – Poutnik Mar 25 at 13:26
• ah.. that means hydroxide also has pKa value of 15.74, and there's no problem with it, Table shows acid side of conjugate bases. is it right? i'm not good at chemistry so it's hard to understand your saying.... so i felt diffcult to say my words maybe... – a wander around chemistry Mar 25 at 13:33