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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?

Hema
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    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. – Avyansh Katiyar Jun 26 '18 at 15:18
  • @AvnishKabaj but many sources say so.. like in this website under Acidity it says so http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch01/ch1-8.html – Hema Jun 26 '18 at 15:20
  • @AvnishKabaj What don't you like about that statement? – Zhe Jun 26 '18 at 15:25
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    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
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    @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. – Avyansh Katiyar Jun 26 '18 at 15:41
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    @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
  • @AvnishKabaj So apparently the issue with this quotation is "it." What is "it?" If "it" is the acid, the statement is correct. If "it" is base, the statement is not. – Zhe Jun 26 '18 at 16:55
  • @Zhe I think so – Avyansh Katiyar Jun 26 '18 at 17:15

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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.

ringo
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