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

To solve the given problem, we need to analyze the chemical reaction between common salt (NaCl), potassium dichromate (\( K_2Cr_2O_7 \)), and concentrated sulfuric acid (\( H_2SO_4 \)). This reaction is known to produce a characteristic gas. Let's understand the reaction step by step:

1. When a mixture of common salt (NaCl) and potassium dichromate (\( K_2Cr_2O_7 \)) is heated with concentrated sulfuric acid (\( H_2SO_4 \)), it produces chromium oxychloride as a gas, also known as chromyl chloride (\( CrO_2Cl_2 \)). The reaction is as follows:

\(4NaCl + K_2Cr_2O_7 + 6H_2SO_4 \rightarrow 2CrO_2Cl_2 + 2KHSO_4 + 4NaHSO_4 + 3H_2O\) 

1. In this reaction, the gas evolved is chromyl chloride (\( CrO_2Cl_2 \)).
2. Let's determine the oxidation state of chromium in the evolved gas, chromyl chloride (\( CrO_2Cl_2 \)):

Chromyl chloride (\( CrO_2Cl_2 \)) has the following structure: \(CrO_2Cl_2: \ Cr + 2\times(-2) + 2\times(-1) = 0\)

Simplifying the equation for oxidation state:

\(Cr - 4 - 2 = 0 \\ Cr = +6\)

1. Thus, the oxidation state of chromium in chromyl chloride (\( CrO_2Cl_2 \)) is +6.
2. Hence, the correct option is that the formula of the gas evolved is \( CrO_2Cl_2 \), and the oxidation state of the central metal atom, chromium, is +6.

Therefore, the correct answer is the first option: \( CrO_2Cl_2 \) and +6.