A) decrease cone. Of \[O{{H}^{-}}\]
B) prevent interference of \[PO_{4}^{3-}\]
C) increase cone. of \[C{{l}^{-}}\]
D) increase cone. 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 cone. of \[O{{H}^{-}},\] as \[N{{H}_{4}}Cl\] and \[N{{H}_{4}}OH\] dissociates as follows. \[N{{H}_{4}}Cl\rightleftharpoons NH_{4}^{+}+C{{l}^{-}}\] \[N{{H}_{4}}OH\rightleftharpoons \underset{\begin{smallmatrix} common \\ \,\,\,\,\,\,\,\,ion \end{smallmatrix}}{\mathop{NH_{4}^{+}+O{{H}^{-}}}}\,\] Due to presence of common ion, cone. of \[O{{H}^{-}}\] decreases and for III group precipitation lesser cone. of \[O{{H}^{-}}\]is required.
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