A) decrease conc. of\[O{{H}^{-}}\]
B) prevent interference of\[PO_{4}^{3-}\]
C) increase conc. of\[C{{l}^{-}}\]
D) increase conc. of\[O{{H}^{-}}\]ion
Correct Answer: A
Solution :
In \[III\] group precipitation, \[N{{H}_{4}}Cl\] is added before adding \[N{{H}_{4}}OH\] to decrease the conc. of\[O{{H}^{-}}\], as \[N{{H}_{4}}Cl\] and \[N{{H}_{4}}OH\] dissociates as follows. \[N{{H}_{4}}ClNH_{4}^{+}+C{{l}^{-}}\] \[N{{H}_{4}}OH\underset{\begin{smallmatrix} common \\ ion \end{smallmatrix}}{\mathop{NH_{4}^{+}+O{{H}^{-}}}}\,\] Due to presence of common ion, conc. of\[O{{H}^{-}}\] decreases and for III group precipitation lesser conc. of \[O{{H}^{-}}\] is required.
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