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: Identification of Compounds A, B and C

Reaction 1 (Chromyl Chloride Test):
Chloride ion reacts with potassium dichromate and concentrated sulphuric acid to form chromyl chloride.
\[ A = CrO_2Cl_2 \]
Reaction 2:
Chromyl chloride reacts with sodium hydroxide to form yellow sodium chromate.
\[ B = Na_2CrO_4 \]
Reaction 3:
Sodium chromate in acidic medium reacts with hydrogen peroxide to form chromium(VI) peroxide, which has a blue colour and butterfly structure.
\[ C = CrO_5 \]

Step 2: Analysis of Structure of C (\(CrO_5\))

The structure of chromium(VI) peroxide consists of one chromium atom bonded to:

• One doubly bonded oxygen atom
• Two peroxo groups (\(O_2^{2-}\)) forming a butterfly structure

Determination of X, Y and Z:

• X (Number of peroxo groups \(O_2^{2-}\)):
  There are 2 peroxo linkages.
  X = 2
• Y (Total number of oxygen atoms):
  From the molecular formula \(CrO_5\).
  Y = 5
• Z (Oxidation state of chromium):
  Let oxidation state of chromium be \(x\).
  \[ x + 1(-2) + 4(-1) = 0 \] \[ x = +6 \] Z = 6

Step 3: Final Calculation
\[ X + Y + Z = 2 + 5 + 6 = \boxed{13} \]