# 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
• @AvnishKabaj but many sources say so.. like in this website under Acidity it says so chem.ucalgary.ca/courses/350/Carey5th/Ch01/ch1-8.html – Hema Jun 26 '18 at 15:20
• Why are these two statements contradictory? If you have a stable conjugate base, then the base is weaker. – Zhe Jun 26 '18 at 15:26
• @Zhe I agree that acids with stable conjugate bases are stronger but over here I think that the op is interpreting that statement as "stronger acids have stronger conjugate bases". That's why I said that the statement is wrong. – Avnish Kabaj Jun 26 '18 at 15:41
• @Hema Your statements don't contradict. A stable conjugate base implies a weak conjugate base. If it is stable, it means it is not as prone to grabbing a proton as an unstable base would be. – Tyberius Jun 26 '18 at 16:20

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.