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
$C{r}_2{O}_7+F{e}^{2+}\to F{e}^{3+}+C{r}^{3+}$
First calculate the oxidation number of $Cr$
$2x+7(-2)=-2$
$2x-14=-2\Rightarrow x=+6$
Oxidation No. increases by $1$ unit (oxidation)
$C{r}_2{O}_7^{2-}+F{e}^{2+}\to F{e}^{3+}+C{r}^{3+}$
In $C{r}_2{O}_7$, the decrease in oxidation number is by $3$ units per atom and since $2$ atoms of chromium are involved so the total decrease is $3\times 2$ $=6$.
$C{r}_2{O}_7+6e^{-}\to C{r}^{3+}$
$F{e}^{2+}\to F{e}^{3+}+e^{-}\times 6$
We need to multiply this equation by $6.$ S
o, the $n$ factor for $C{r}_2{O}_7=6$ and for $F{e}^{2+}=1$.
So, to reduce $1mol$ of dichromate, $6$ moles of $F{e}^{2+}$ are required.