The oxidation state of $\mathrm{Cr}$ in $\mathrm{CrO}{}_5$ is

Solution & Explanation

\textbf{Answer:} (c) $+6$. \textbf{Solution:} $\mathrm{CrO}{}_5$ (chromium peroxide, also written as $\mathrm{CrO}(\mathrm{O}{}_2){}_2$) is a peroxo species containing two peroxo linkages ($\mathrm{O}-\mathrm{O}$). In these four peroxo oxygens the oxidation state is $-1$, while the one doubly-bonded oxygen is $-2$. Let $\mathrm{Cr}$ $=x$: $x+4(-1)+1(-2)=0\Rightarrow x=+6$. Hence $\mathrm{Cr}$ is in the $+6$ state. (If all five oxygens were counted as $-2$ one would wrongly get $+10$; the peroxo bonding is the key.)