A) 4 and 5
B) 4 and 8
C) 8 and 4
D) 8 and 9
Correct Answer: C
Solution :
[c] \[C{{r}_{2}}O_{4}^{2-}\] ions are invariably in equilibrium with \[Cr{{O}_{4}}^{2-}\] ions in aqueous solution at pH=4. \[\underset{(Yellow)}{\mathop{CrO_{4}^{2-}}}\,\underset{O{{H}^{-}}}{\overset{{{H}^{+}}}{\longleftrightarrow}}\underset{(Orange\,\,red)}{\mathop{C{{r}_{2}}O_{7}^{2-}}}\,\] On increasing the \[pH\]of solution (adding alkali), the \[{{H}^{+}}\]ions are used up and according to law of chemical equilibrium, the reaction processed in the forward direction, producing yellow chromate solution. If pH of solution is decreased (acid is added), conc. of \[{{H}^{+}}\] ions increased, thus producing orange red dichromate solution. \[C{{r}_{2}}O_{7}^{2-}\xrightarrow{pH=\,8}CrO_{4}^{2-}\xrightarrow{pH=\,4}C{{r}_{2}}O_{7}^{2-}\]
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