H₂O₂ changes Cr₂O₇²⁻ ion to CrO₅ in an acidic medium, the oxidation state of Cr in CrO₅ is

The correct option is +6.

To determine the oxidation state of chromium (Cr) in chromium pentoxide (${\mathrm{CrO}}_5$), we need to examine its chemical structure. Applying the standard oxidation state rules directly (assuming all oxygen atoms have an oxidation state of -2) would incorrectly yield +10, which is impossible since chromium only has 6 valence electrons.

Structure of ${\mathrm{CrO}}_5$:
Chromium pentoxide has a butterfly structure that features peroxide linkages:
1. One oxygen atom is double-bonded to chromium ($\mathrm{Cr}=O$). This is an oxo-oxygen atom with a standard oxidation state of -2.
2. The other four oxygen atoms form two peroxide rings ($-O-O-$). Each oxygen atom in a peroxide linkage has an oxidation state of -1.

Calculating the Oxidation State of Chromium:
Let $x$ be the oxidation state of chromium in ${\mathrm{CrO}}_5$. Since the overall charge of the molecule is neutral (0), we can set up the following equation:

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

Solving for $x$ step-by-step:

$x-2-4=0$

$x-6=0$

$x=+6$

Therefore, the oxidation state of chromium in ${\mathrm{CrO}}_5$ is +6.