PKa values for weak/strong bases

[kasse]

In chemistry it's normal to distinguish between weak and strong acids. Is there a limit for the pK_a value that determines what is strong and what is weak?

Equivalent, for bases, is there a pK_b value that separates weak and strong bases?

 

[Borek]

As a first approximation you may assume that everything with pKa<1 is strong, and everything with pKa>3 is weak (symmetrically for bases). Between 1 and 3 is a grey area. But if now someone will answer telling you that strong is below 0 and weak is above 2 - s/he will be right as well. There is no set limit, as these things tend to be somewhat relative.

 

[epenguin]

And of course we mostly think in terms of water solvent. But in liquid ammonia solvent anything stronger than acetic acid is 'strong' and fully dissociated, while in acetic acid solvent HCl and HNO3 are fully associated and perchloric acid is one of the few remaining dissociated, so 'strong'. This is called a 'levelling' effect.

It depends on solvation effects and (not always) oproton donation to /acceptance from the medium.
These effects are absent in the gas phase where all molecules are extremely weak acids. But there e.g. the water molecule is a weaker acid (proton donor) than toluene!
You will mostly deal with water medium but just a little relativisation.