Question

Compare the basic strength of the following:
(a) $H_{2}P{O}_4{}^{-}$, $H_{2}P{O}_3{}^{-}$
(b) $HP{O}_4^{2-}$, $HP{O}_3^{2-}$
(c) $Cl{O}^{-}$, $Cl{O}_2{}^{-}$
(d) $N{H}_3$, $N{H}_2{}^{-}$
Which among the following is/are correct?

• A. $H_{2}P{O}_4{}^{-}$ < $H_{2}P{O}_3{}^{-}$
• B. $HP{O}_4^{2-}$ $<$ $HP{O}_3^{2-}$
• C. $Cl{O}^{-}$ $>$ $Cl{O}_2{}^{-}$
• D. $N{H}_3$ $<$ $N{H}_2{}^{-}$

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A $HP{O}_4^{2-}$ $<$ $HP{O}_3^{2-}$
B $Cl{O}^{-}$ $>$ $Cl{O}_2{}^{-}$
C $N{H}_3$ $<$ $N{H}_2{}^{-}$
D $H_{2}P{O}_4{}^{-}$ < $H_{2}P{O}_3{}^{-}$

More the resonating structure (canonical forms), lesser the stability of conjugate base. Hence, availability of lone pair electrons to be donated will be less, and hence basic strength will also be less. The order of stability is as follows: $H_{2}P{O}_4^{-}>H_{2}P{O}_3^{-}$ so order of basic strength is $H_{2}P{O}_4^{-}<H_{2}P{O}_3^{-}$ .
Another anion of oxoacids in b, c, also have the same concept.
In $D$, as $N{H}_2^{-}$ is charged one, so it has more availability of electrons to donate, hence, more basic than $N{H}_3$.