Question

On heating a mixture of common salt and \( K_2Cr_2O_7 \) in equal amount along with concentrated \( H_2SO_4 \) in a test tube, a gas is evolved. Formula of the gas evolved and oxidation state of the central metal atom in the gas respectively are :

• A. \( CrO_2Cl_2 \) and +6
• B. \( CrO_2Cl_2 \) and +5
• C. \( Cr_2O_2Cl_2 \) and +3
• D. \( Cr_2O_2Cl_2 \) and +6

Hint:

Chromyl chloride vapors dissolve in NaOH to form a yellow solution of sodium chromate, confirming the presence of chloride.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This is the "Chromyl Chloride Test," used to detect the presence of chloride ions in a sample.

Step 2: Key Formula or Approach:
The reaction is:
\( K_2Cr_2O_7 + 4NaCl + 6H_2SO_4 \xrightarrow{\Delta} 2KHSO_4 + 4NaHSO_4 + 2CrO_2Cl_2 \uparrow + 3H_2O \)

Step 3: Detailed Explanation:
When common salt (\( NaCl \)) and potassium dichromate are heated with concentrated sulfuric acid, deep red vapors of chromyl chloride (\( CrO_2Cl_2 \)) are evolved.
In \( CrO_2Cl_2 \), let the oxidation state of Chromium be \( x \).
Oxygen has an oxidation state of -2, and Chlorine has an oxidation state of -1.
\( x + 2(-2) + 2(-1) = 0 \Rightarrow x - 4 - 2 = 0 \Rightarrow x = +6 \).
The oxidation state of the central metal (Cr) is +6.

Step 4: Final Answer:
The gas is \( CrO_2Cl_2 \) and the oxidation state is +6.