Question

Consider the reactions: 1. \(NaCl+K_{2}Cr_{2}O_{7}+H_{2}SO_{4} \rightarrow A + ...\) 2. \(A + NaOH \rightarrow B + ...\) 3. \(B + H_{2}SO_{4}+H_{2}O_{2} \rightarrow C + ...\) In the product 'C', X is the number of \(O_{2}^{2-}\) units, Y is the total number of oxygen atoms, and Z is the oxidation state of Cr. The value of \(X+Y+Z\) is__.
  

Hint:

\(CrO_5\) is unstable and decomposes to \(Cr^{3+}\) and oxygen if not stabilized by organic solvents like ether. The butterfly structure is a classic exam favorite.

Answer 1

Correct Answer: 18

   Step 1: Identify Compounds A, B, C
\begin{itemize} \item Reaction 1 (Chromyl Chloride Test): Chloride + Dichromate + Conc. Acid forms Chromyl Chloride (\(CrO_2Cl_2\)). \[ A = CrO_2Cl_2 \] \item Reaction 2: Chromyl Chloride reacts with NaOH to form Sodium Chromate (Yellow). \[ B = Na_2CrO_4 \] \item Reaction 3: Chromate in acidic medium with Hydrogen Peroxide forms Chromium(VI) Peroxide (Blue butterfly structure). \[ C = CrO_5 \] \end{itemize} Step 2: Analyze Structure of C (\(CrO_5\))
Structure consists of one Chromium atom double bonded to one Oxygen, and bonded to two Peroxo groups (\(O_2^{2-}\)) in a butterfly shape. \begin{itemize} \item X (Number of \(O_2^{2-}\) units): There are 2 peroxy linkages/rings. \(X = 2\). \item Y (Total Oxygen atoms): Formula is \(CrO_5\). \(Y = 5\). \item Z (Oxidation State of Cr): The oxidation state is \(+6\) (Calculated as \(x + 1(-2) + 4(-1) = 0 \implies x = 6\)). \(Z = 6\). \end{itemize} Step 3: Calculation
\[ X + Y + Z = 2 + 5 + 6 = 13 \]