How does pKa relate to basicity?
The above equation relates to aqueous solutions only. I think you know that for strong acids, (e.g. hydrogen halides, sulfuric, perchloric acid etc.) the equlibium lies strongly to the RIGHT as written:
So strong acids tend to donate a proton to the solvent molecule (which here is water). This is equivalent to the argument that the conjugate base of the acid, here
On the other hand, we could display the acid-base behaviour of hydrogen fluoride, a weak to moderate Bronsted acid:
Here, the equilibrium lies to the LEFT (certainy further to the left than for the other hydrogen halides where there is poor overlap between hydrogen and halide, why?)
So the take home message? Strong acid; weak conjugate base. Weak acid; strong conjugate base.