Question

$K_b$ values for the aqueous methylamine, ammonia, urea and pyridine are: $5.6\times {10}^{-4}$, $1.8\times {10}^{-5}$, $1.5\times {10}^{-14}$ and $1.7\times {10}^{-9}$ respectively.
Which is the correct order of basic strength?

• A. Methylamine > Ammonia > Urea > Pyridine
• B. Methylamine < Ammonia < Urea < Pyridine
• C. Methylamine > Ammonia > Pyridine > Urea
• D. Methylamine < Ammonia < Pyridine < Urea

Answer 1

The correct option is C Methylamine > Ammonia > Pyridine > Urea

$K_b$ is the base dissociation constant.
It is a measure of how completely a base dissociates into its component ions in water. The numerical value of $K_b$ is a reflection of the strength of the base. Strong bases have high base dissociation constant $K_b$ value and weak bases have very low $K_b$ value.

The increasing order of $K_b$ value is Methylamine($5.6\times {10}^{-4})$ > Ammonia$(1.8\times {10}^{-5})$ > Pyridine$(1.7\times {10}^{-9})$ > Urea$(1.5\times {10}^{-14})$
More the $K_b$ value more stronger is base. hence correct basic order is Methylamine > Ammonia > Pyridine > Urea