EK says Weak acid Conjugate Strong Base! (necessarily)...

[SaintJude]

So I'm reading EK, and they write:

"The stronger the acid, the weaker its conjugate base and the stronger the base, the weaker its conjugate acid. Warning: Many students translate this into "Strong acids have weak conjugate bases and weak acids have strong conjugate bases." The second part of this statement is incorrect! Acid strength is on the logarithmic scale and a weak acid may have a strong or weak conjugate base. "

I included the EK graph that accompanied this statement
View attachment Picture 35.png
Can someone explain a bit further what EK means? b/c it's these type of common misconception that MCAT often tests...
Thanks!

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[mcloaf]

TBR emphasizes this as well. The point is that this reasoning is one directional. For an acid or base which is strong, you can judge the weakness of its conjugate accordingly. This is the standard idea of "acid HX is stronger than acid HY, so base X is weaker than base Y."

BUT, you can't go in the reverse direction and say that because something is a weak acid or base, its conjugate will therefore be strong. Most of the time the conjugate of a weak acid or base will also be weak.

 

[SaintJude]

Mhh, still would like to go deeper. Does anyone understand the actual chemistry behind this? Or have an example of a weak acid that has a weak conjugate base?

 

[MedPR]

Mhh, still would like to go deeper. Does anyone understand the actual chemistry behind this? Or have an example of a weak acid that has a weak conjugate base?

Conjugate pKa+pKb = 14. Take HClO. I think its pKa is around 7, that means the pKb for ClO- must be around 7 also. Both are weak acid and base.

 

[milski]

Take the pic as an example: A is strong acid and its conjugate base will be weak. The conjugate base of C is a strong base and that means C will be a weak acid.

But B is a weak acid but not weak enough to make its conjugate base strong. So for weak acids, you have both strong conjugate bases (C) and weak conjugate bases (B).

 

[SaintJude]

Yes, and an example is NH3 and NH4+! NH4 is a weak acid that has a weak conjugate base: NH3. I'm surprised it too me so long to recall an example. Thank you for all your replies!