Ionization constant of a weak base MOH, is given by the expression
Kb = 
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?
pKb values for Di-methylamine, ammonia, pyridine and urea are respectively 3.29, 4.752, 8.752, and 13.8861.
The value of Kb (base ionisation constant) defines the strength of a base. Weak bases with relatively higher values are strong that bases with lower kb values.
The decreasing order of bases on the basis of 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.