Question

Consider a titration of potassium dichromate solution with acidified Mohr's salt solution using diphenylamine as indicator. The number of moles of Mohr's salt required per mole of dichromate is

• A. 3
• B. 4
• C. 5
• D. 6

Answer 1

Answer: (D)

The skeletal ionic equation is
$Cr _{2} O _{7}+ Fe ^{2+} \rightarrow Fe ^{3+}+ Cr ^{3+}$
First calculate the oxidation number of $Cr$
$2 x+7(-2)=-2 $
$2 x-14=-2 \Rightarrow x=+6$
Oxidation No. increases by $1$ unit (oxidation)
$Cr _{2} O _{7}^{2-}+ Fe ^{2+} \rightarrow Fe ^{3+}+ Cr ^{3+}$
In $Cr _{2} O _{7}$, the decrease in oxidation number is by $3$ units per atom and since $2$ atoms of chromium are involved so the total decrease is $3 \times 2$ $=6$.
$ Cr _{2} O _{7}+6 e^{-} \rightarrow Cr ^{3+}$
$Fe ^{2+} \rightarrow Fe ^{3+}+e^{-} \times 6$
We need to multiply this equation by $6 .$ S
o, the $n$ factor for $Cr _{2} O _{7}=6$ and for $F e^{2+}=1$.
So, to reduce $1 \,mol$ of dichromate, $6$ moles of $F e^{2+}$ are required.