A) \[N{{H}_{4}}A<N{{H}_{4}}B<N{{H}_{4}}C\]
B) \[N{{H}_{4}}C<N{{H}_{4}}B<N{{H}_{4}}A\]
C) \[N{{H}_{4}}C<N{{H}_{4}}A<N{{H}_{4}}B\]
D) All have equal pH
Correct Answer: B
Solution :
\[\because \] Salts \[N{{H}_{4}}A,\,N{{H}_{4}}B,\,N{{H}_{4}}C)\] are of weak acid and weak base. \[\therefore \] \[pH=7+\frac{p{{K}_{a}}}{2}-\frac{p{{K}_{a}}}{2}\] Thus, greater the value of \[p{{K}_{a}}\] of HA, greater the \[pH\]. \[p{{K}_{b}}({{A}^{-}},{{B}^{-}},{{C}^{-}})|\underset{4}{\mathop{{{A}^{-}}}}\,\,\,<\,\,\underset{5}{\mathop{{{B}^{-}}}}\,\,\,<\underset{6}{\mathop{{{C}^{-}}}}\,\] \[\because p{{K}_{a}}\,(HA,\,HB,\,HC)|\,\underset{8}{\mathop{HC}}\,<\underset{9}{\mathop{HB}}\,<\underset{10}{\mathop{HA}}\,\] Thus, order of pH values is \[(N{{H}_{4}}C<N{{H}_{4}}B<N{{H}_{4}}A)\].
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