Question
Correct statements from the following are
A. Potassium dichromate is an oxidizing agent and it oxidizes ${FeSO}_{4}$ to ${Fe}_{2}\left( {SO}_{4} \right)_{3}$ in acidic medium.
B. Sodium dichromate can be used as primary standard in volumetric estimation.
C. ${CrO}_{4}^{2 -}$ and ${Cr}_{2}O_{7}^{2 -}$ are interconvertible in aqueous solution by varying the pH of the solution.
D. $Cr - O - Cr$ bond angle in ${Cr}_{2}O_{7}^{2 -}$ is $126^{\circ}$.
Choose the correct answer from the options given below:
(1) A, B and C only
(2) A, C and D only
(3) A and C only
(4) B and D only
Solution
Sol. [Key Concept: $K_{2}{Cr}_{2}O_{7}$ oxidises ${Fe}^{2 +}$ (A); ${CrO}_{4}^{2 -} \rightleftharpoons \ {Cr}_{2}O_{7}^{2 -}$ is pH-controlled (C); Cr–O–Cr angle $126^{\circ}$ (D) — but ${Na}_{2}{Cr}_{2}O_{7}$ is deliquescent (B false).]
(A) ${MnO}_{4}^{-} + {Fe}^{+ 2} + H^{+} \longrightarrow {Fe}^{+ 3} + {Mn}^{+ 2} + H_{2}O$
(B) ${Na}_{2}{Cr}_{2}O_{7}$ generally not used as a primary standard in volumetric analysis because it is deliquescent.
(C) In acidic medium ${Cr}_{2}O_{7}^{2 -}$ exists and it get converted into ${CrO}_{4}^{2 -}$ in basic medium.
$${2{CrO}_{4}^{2 -} + 2H^{+} \longrightarrow {Cr}_{2}O_{7}^{2 -} + H_{2}O }{{Cr}_{2}O_{7}^{2 -} + 2{OH}^{-} \longrightarrow 2{CrO}_{4}^{2 -} + H_{2}O }$$(D) $K_{2}{Cr}_{2}O_{7}$ is a good oxidising agent and bond angle of $Cr - O - Cr$ bond in ${Cr}_{2}O_{7}^{2 -}$ is $126^{\circ}$
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