Question

describe the relation between pKa and pKb - detailed answer ​

Answer 1

hyyy here is your answer. .....

A large Kb value indicates the high level of dissociation of a strong base. A lower pKb value indicates a stronger base. pKa and pKb are related by the simple relation: pKa + pKb = 14.

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Answer 2

The pKa of NH3 is 38. Does it stand for the following equilibrium:

NH4+↽−−⇀NH3+H+(1)

With Ka=10−38 for reaction (1), indicating that the conjugate acid of NH3 is quite weak?

The pKb of NH3 is 4.74. Does it stand for the following equilibrium:

NH3+H2O↽−−⇀NH4++OH−(2)

With Kb=10−4.74 for reaction (1), indicating that NH3 is a weak base?

Also:

pKa+pKb=42.74≠14

The pKa of NH3 is 38.

This means, the reaction this value is describing is:

NH3↽−−⇀NH2−+H+

The pKb of NH3 is 4.74.

This means, the reaction this value is describing is:

NH3+H+↽−−⇀NH4+

There is a lot of misuse going on with the term pKa since many people are too lazy to say

The pKa of NH3’s conjugate acid is 9.26

NH4+↽−−⇀NH3+H+

rather than

The pKa of NH3 is 9.26.

NH3↽−−⇀NH2−+H+

but that is really what they should be doing. Thankfully, when looking up values in a table the person compiling the table (hopefully) knew what they were doing and used the correct terminology.

Only for a pKa and a pKb that describe the same reaction (from opposite viewpoints) is it true that pKa+pKb=14.