In chromyl chloride test for confirmation of Cl- ion, a yellow solution is obtained. Acidification of the solution and…

Step 1: Understand the context

The question refers to the formation of chromium pentoxide ($\ce{CrO5}$), which is known for its deep blue color extracted into organic layers like amyl alcohol. This compound is formed when chromates or dichromates react with hydrogen peroxide in an acidic medium.

Step 2: Determine the structure of Chromium Pentoxide ($\ce{CrO5}$)

Chromium pentoxide ($\ce{CrO5}$) has a unique "butterfly" structure. It consists of:

• One central Chromium atom.
• One double-bonded oxygen atom (oxide).
• Four oxygen atoms in two peroxide ($\ce{-O-O-}$) linkages.

Step 3: Calculate the oxidation state

Let the oxidation state of Cr be $x$.

• The oxide oxygen contributes $-2$.
• The four peroxide oxygen atoms each contribute $-1$.

The sum of the oxidation states must equal the overall charge of the neutral molecule ($0$): $x + (-2) + 4(-1) = 0$ $x - 2 - 4 = 0$ $x = +6$

Step 4: Conclusion

The maximum oxidation state of chromium cannot exceed its group number ($VI$ or 6). Thus, calculating $x = +10$ from the empirical formula without considering peroxides is incorrect. The true oxidation state is $+6$.

Key Points to Remember:

• The blue color formed when $\ce{H2O2}$ is added to an acidified chromate/dichromate is due to $\ce{CrO5}$.
• $\ce{CrO5}$ is stabilized in organic solvents like amyl alcohol or ether.
• $\ce{CrO5}$ contains two peroxide bonds and one oxide bond, meaning the oxidation state of Cr remains $+6$.