How to find pKa from pKb

PKs and pKb value

At the pKs and the pKb these are values ​​that indicate how strong an acid or base is. Every acid has a characteristic pKs Value, while each base has a characteristic pKb Has.
These values ​​are defined using the negative decadic logarithm of the Ks- and Kb-Values.


Adding the two values ​​(pKs and pKb) of an acid / conjugate base pair, they always result in a sum of 14. This can be done with the equations for the Ks- and the Kb-Explain the value of which pKb and pKs depend.


If you multiply these two equations, you get the following equation after reduction:

This equation in turn corresponds to the water constant K.wwhich at room temperature always 10-14 Mole2 * l-2 amounts to.
Accordingly:

By using the negative decadic logarithm and the definition of the pKb and pKsValue one arrives at the equation: pKs + pKb = 14
Since a strong acid has a high Ks value, the pKa value of a strong acid is correspondingly low and vice versa.
There are the following classifications:
pKs<1.5 -> Strong acid
1.5 s<4.75 -> Medium acidity
pKs> 4.75 -> weak acid

This applies analogously to the pKb and the base strength.
In general, every connection with a pKs or pKb under 14 accordingly react acidic or basic. If both values ​​are in this range for an acid / conjugate base pair, one speaks of an ampholyte, a substance that can react both acidic and basic.