[DonaldGlover]

Whats the easiest way to know it? Like we all know pH 1 is acid and 14 is alkaline. But what is it for pKa?

[EierVonSatan]

They're slightly different, really.

pH is a measure of much H+/OH- there is in solution (how acidic/basic a solution is, if you will). It varies on concentration.

pK_a on the other hand is a measure of how acidic a molecule is (the ability to lose H+) and won't vary with concentration of a solution it's in.

If you look up the pK_a values for various types of aqueous acids (or what we call acids) then strong acids like HCl and H₂SO₄ will have negative values. Weak acids like ethanoic acid will be around 5, phenol about 10.

If you want to think of a chemical that is neutral in water - say propanone - it's pK_a is 25. A very high pK_a (such as hexane pK_a ~50) does not mean it's basic however, just that it's not at all acidic.