Question

The correct decreasing order of basic strength of the following species is:
 $ {H_2}O,N{H_3},O{H^ - },N{H_2}^ - $
(A) $ N{H_2}^ - > O{H^ - } > N{H_3} > {H_2}O $
(B) $ O{H^ - } > N{H_2}^ - > {H_2}O > N{H_3} $
(C) $ N{H_3} > {H_2}O > N{H_2}^ - > O{H^ - } $
(D) $ {H_2}O > N{H_3} > O{H^ - } > N{H_2}^ - $

Answer 1

Hint: The basic strength means the capability of donating electrons or the capability of abstraction of a proton. The presence of lone pairs of electrons in atoms makes the molecule to be more basic. Ammonia is more basic than water. Thus, the conjugate base of ammonia is more basic than the conjugate base of water.

Complete answer:
Water is a molecule that contains an oxygen atom and two hydrogen atoms. Oxygen has two lone pairs of electrons in water molecules, which makes the water basic, but actually water is a neutral compound.
Due to the presence of lone pairs of electrons on nitrogen in ammonia, ammonia can be considered as a base. But, due to the low electronegativity value of nitrogen compared to oxygen. Ammonia is more basic than water.
The conjugate base of ammonia is amine ion, it is more basic than ammonia. The conjugate base of water is hydroxide ion, it is more basic than water.
The group $ N{H_2}^ - $ is more basic in all the given molecules or groups.
Thus, the order of decreasing basic strength is $ N{H_2}^ - > O{H^ - } > N{H_3} > {H_2}O $
Option A is the correct one.

Note:
The compounds that will release a proton can be called an acid. The group that can have one less number of hydrogen atoms with a negative charge to its acid, can be considered as a conjugate base. Conjugate base of ammonia $ \left( {N{H_3}} \right) $ is $ N{H_2}^ - $ , similarly the conjugate base of water $ \left( {{H_2}O} \right) $ is $ O{H^ - } $ .