# Is an acid strong when its conjugate base is strong or weak? [duplicate]

It is often said that stronger acids have weak conjugate bases and vice versa. Yet also in organic chemistry if an acid is proven to have a stable conjugate base it is said to dissociate more, which means it is stronger. Aren't these contradictory statements? What exactly should a strong acid have- a strong or a weak conjugate base?

## marked as duplicate by Mithoron, Todd Minehardt, Jon Custer, jerepierre, Community♦Jun 28 '18 at 12:57

• if an acid is proven to have a stable conjugate base it is said to dissociate more, which means it is stronger This is wrong. – Avnish Kabaj Jun 26 '18 at 15:18
Since $\mathrm{pK_a+pK_b=14}$ and strong acids are defined as dissociating completely in water, then a strong acid must be stronger than hydronium, meaning its $\mathrm{pK_a<-1.74}$. For its conjugate base then, this means $\mathrm{pK_b>15.74}$. So yes, a strong acid and a weak conjugate base are two sides of the same coin.