Conjugate acids/bases

[Bailey Herbert 2L]

Is the conjugate base of a strong acid always weak? And vice vera (strong acid, weak conjugate base)? Or can you have a weak acid with a weak conjugate base? Why or why not?

[105605391]

The conjugate of a strong acid or strong base is always weak. To better understand this principle it is helpful to understand what makes an acid or base strong, that being the complete deprotonation or protonation of the species respectively. If something is completely dissociating an H+ or accepting one then the conjugate that was once bonded was bonded really weakly making it less likely to pull hydrogen back or release one depending on it being an acid or a base respectively. Hope this helps!

[Cassidy Chiong 2J]

Yes, the conjugate base of a strong acid is always weak, and the conjugate acid of a strong base is always weak.

Take HCl for example. It has a really high Ka of 1.3 * 10^6, meaning that it is more like to dissociate into H+ and Cl- ions since Ka=[H+][Cl-]/[HCl]. Thus, if it is more likely to exist in the ionic form, Cl- (the conjugate base of HCl) does not have as strong of a desire to regain that H+ ion that it lost.

It also helps to think in terms of Ka and Kb. The Ka of HCl is really high (1.3 * 10^6). If you take the reciprocal to find the Kb of Cl-, you'd get 7.7 * 10^-7, which is really small. This small value means that Cl- is more likely to exist as Cl- rather than act as a base and gain an H+ ion to form HCl.

[Sevde Coban 2J]

Yes, the stronger the acid, the weaker its conjugate base is. You can use Kw = Ka x Kb to understand this. If the Ka of an acid is large, then its conjugate base will have a small Kb, suggesting that base is weak.

[SofiaMammaro-1K]

A strong acid has a weak conjugate base, whereas a weak acid has a relatively strong conjugate base because a strong acid necessarily implies a weak conjugate base, and a strong base implies a weak conjugate acid.

[Nomi Heidari-Bateni 2K]

Yes, you are correct. A strong acid has a weak conjugate base and a strong base has a weak conjugate acid. I believe the stronger the acid, the weaker its conjugate base always, and vice versa. We can analyze this mathematically since we know that a low dissociation constant (K) means lower strength while a high one means higher strength.
Kw (a constant) = Ka x Kb
Thus, if Ka is high (strong acid) then Kb must be low (weak base) to make this equation true.