Question

A white coloured compound 'X' heated strongly to form white powder of 'PbO' along with two gases 'Y' : brown in colour, 'Z' : colorless. Identify 'X', 'Y', 'Z'.

• A. \(X = \text{Pb(NO}_3)_2\) ; \(Y = \text{NO}_2\) ; \(Z = \text{O}_2\)
• B. \(X = \text{Pb(NO}_3)_2\) ; \(Y = \text{NO}\) ; \(Z = \text{O}_2\)
• C. \(X = \text{Pb(NO}_2)_2\) ; \(Y = \text{NO}\) ; \(Z = \text{O}_2\)
• D. \(X = \text{Pb(NO}_2)_2\) ; \(Y = \text{NO}_2\) ; \(Z = \text{NO}_3\)

Hint:

In high school chemistry, "brown fumes" or "brown gas" during thermal decomposition is almost always nitrogen dioxide (\(\text{NO}_2\)).
This simple clue helps you isolate the correct option immediately.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question describes a thermal decomposition reaction of a white compound 'X' that decomposes into a yellow/white residue of Lead Oxide (PbO) and two gases: a brown gas 'Y' and a colorless gas 'Z'. We need to identify these substances.

Step 2: Key Formula or Approach:
Recall the thermal decomposition reaction of metal nitrates, specifically Lead Nitrate.

Step 3: Detailed Explanation:

• Lead nitrate, \(\text{Pb(NO}_3)_2\), is a white crystalline solid.

• Upon strong heating in a test tube, lead nitrate decomposes into lead monoxide, nitrogen dioxide gas, and oxygen gas.

• The balanced chemical equation for this thermal decomposition is:
\[ 2\text{Pb(NO}_3)_2\text{(s, white)} \xrightarrow{\Delta} 2\text{PbO(s, yellow/white)} + 4\text{NO}_2\text{(g, brown fume)} + \text{O}_2\text{(g, colorless)} \] - The gas 'Y' with a characteristic brown color is nitrogen dioxide (\(\text{NO}_2\)).
- The gas 'Z' which is colorless and supports combustion is oxygen (\(\text{O}_2\)).

• Comparing with the choices, Option (A) is the correct match.


Step 4: Final Answer:
The correct identification is \(X = \text{Pb(NO}_3)_2\), \(Y = \text{NO}_2\), \(Z = \text{O}_2\).