Reaction with pKas?

Question

In my solutions manual, a question is explained like this:
To react completely with NaOH, an acid must have a pKa at least 3 units smaller than the pKa of H2O (it's conjugate acid).

Is the 3 unit smaller thing true for all reactions that goes to 99.9% completion, or is it only specific for this particular reaction with NaOH?
Thanks!

Answer 1

This is only true for weak acid-strong base reactions. Basically, if the acid is "too weak", the equilibrium will prevent total conversion of the acid to the acid cation (the H+ reacts with the OH- from NaOH, and the cation balances out the Na+).