Question

how to express the value of pka and pkb for any organic compounds.

Answer 1

Ka= dissociation constant of acid; Kb= dissociation constant of base

pKa and pKb are measures of the strengths of acids and bases, respectively

Acids:
When you dissolve an acid in water, it undergoes an equilibrium reaction with the water

HA + H₂O ⇌ H₃O⁺ + A⁻

The value of the equilibrium constant is given by

Ka= [H3O+][A−] /HA

The greater the value of Ka, the stronger the acid.

For most weak acids, Ka ranges from 10−2 to 10−14.

We convert these exponential numbers into a normal range by taking their negative logarithm.

The operator p means "take the negative logarithm of".

So pKa=−logKa.

For most weak acids, pKa ranges from 2 to 14.

Thus, the smaller the value of pKa , the stronger the acid.

Bases:

When you dissolve a base in water, it reacts with the water in an equilibrium reaction.

B + H₂O ⇌ BH⁺ + OH⁻

The value of the equilibrium constant is given by

Kb=[BH+][OH−]/B

The greater the value of Kb, the stronger the base.

For most weak acids, Kb ranges from 10−2 to 10−13.

pKb=−logKb.

For most weak acids, pKa ranges from 2 to 13.

The smaller the value of pKb , the stronger the base.