Question

What is the oxidation state of Chromium ($Cr$) in $Cr{O}_5$ ?

Answer 1

$Cr{O}_5$ contains two peroxo linkages ($O-O$ bond) in which each O atom has an oxidation number of $-1.$
Oxygen atom bonded with Cr by double bond has oxidation number of $-2.$

Let the oxidation state of $Cr$ in $Cr{O}_5$ be $x.$

$x+(-2)+4(-1)=0$

$x=+6$

Hence, the oxidation state of $Cr$ in $Cr{O}_5$ is $+6.$