Question

An element $A$ in a compound $ABD$ has oxidation number $A^{n-}$. It is oxidised by $C{r}_2{O}_7^{2-}$ in acid medium. In the experiment, $1.68\times {10}^{-3}$ moles of $K_{2}C{r}_2{O}_7$ were used for $3.26\times {10}^{-3}$ moles of $ABD$. The new oxidation number of $A$ after oxidation is:

• A. $3$
• B. $3-n$
• C. $n-3$
• D. $+n$

Answer 1

The correct option is B $3-n$

$Cr$ changes its oxidation state from $+6$ to $+3$ in acidic medium on oxidizing a compound.

So, the net change in oxidation per molecule of $K_{2}C{r}_2{O}_7$ is $+6$ ($2$ $Cr$ atoms per molecule).

As $1$ equivalent of $K_{2}C{r}_2{O}_7$ oxidizes two equivalents of $ABD$, then each $A$ atom changes its oxidation state by $3$.
So, the new oxidation state is $-n+3=3-n$.

Hence, the correct option is $C$