A) It confirms the presence of \[C{{l}^{-}}\] ion
B) It confirms the presence of \[B{{r}^{-}}\] ion
C) It confirms the presence of both
D) It neither confirms \[C{{l}^{-}}\] nor \[B{{r}^{-}}\] unless it is passed through \[NaOH\] solution
Correct Answer: D
Solution :
When\[C{{l}^{-}}\]salt is heated with\[{{K}_{2}}C{{r}_{2}}{{O}_{7}}+\]conc.\[{{H}_{2}}S{{O}_{4}}\]red vapours of \[Cr{{O}_{2}}C{{l}_{2}}\]are evolved which on passing into NaOH solution gives yellow solution of \[Cr{{O}_{4}}^{2-}\]ions. Latter further gives yellow ppt. of \[PbCr{{O}_{4}}\]with \[Pb(C{{H}_{3}}CO)2\]and acetic acid. \[4C{{l}^{-}}+C{{r}_{2}}O_{7}^{--}+6{{H}^{+}}\xrightarrow[{}]{{}}\underset{\text{reddish}\,\text{brown}\,\text{vapours}}{\mathop{2Cr{{O}_{2}}Cl\uparrow +3{{H}_{2}}O}}\,\] \[Cr{{O}_{2}}C{{l}_{2}}+4O{{H}^{-}}\xrightarrow[{}]{{}}\underset{\text{Yellow}\,\text{so}{{\text{l}}^{\text{n}}}.}{\mathop{CrO_{4}^{--}2C{{l}^{-}}}}\,+2{{H}_{2}}O\] While on heating a bromide salt with conc.\[{{H}_{2}}S{{O}_{4}}\And {{K}_{2}}C{{r}_{2}}{{O}_{7}}\], instead of \[Cr{{O}_{2}}B{{r}_{2}}\], reddish brown fumes of \[B{{r}_{2}}\]are formed due to reducing character of HBr formed during the reaction. \[B{{r}_{2}}\]dissolves in NaOH & No colour is produced. \[6B{{r}^{-}}+C{{r}_{2}}O_{7}^{--}+{{H}^{+}}\xrightarrow[{}]{{}}3B{{r}_{2}}\underset{\text{Reddish}\,\text{brown}\,\text{vapours}}{\mathop{\uparrow +C{{r}^{3+}}+7{{H}_{2}}O}}\,\]
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