Ionization constant of a weak base

Question:

Ionization constant of a weak base MOH is given by the expression

Kb = [M+][OH-]/[MOH-]

Values of ionisation constant of some weak bases at a particular temperature are given below:

Base Di-methylamine Urea Pyridine Ammonia

Kb 5.4 × 10-4 1.3 × 10-14 1.77× 10-9 1.77 × 10-5

Arrange the bases in decreasing order of the extent of their ionisation at Equilibrium. Which of the above base is the strongest?

Solution:

The decreasing order of bases based on the ionisation constant at Equilibrium will be;-

Di-methylamine > Ammonia > Pyridine > Urea

The strongest base will be Di-methyl amine as its pKb value is 3.29 and we know that the less the pKb value strong is the base.

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