`M("Salt")+"Dil. "HCl overset(Warm)to P+Q uarr+H_(2)O`
gas 'Q' changes colour of `FeSO_(4)` solution into yellow solution then salt M in above reaction is

Options

A. `BaS_(2)O_(3)`
B. `Ag_(2)SO_(3)`
C. `AgNO_(2)`
D. `Pb(NO_(3))_(2)`

Explanation

To determine the salt M in the reaction with dilute hydrochloric acid that produces a gas Q, which changes the color of FeSO4 solution to yellow, we can follow these steps:

### Step 1: Understand the Reaction
We have a salt M that reacts with dilute HCl upon warming to produce products P, Q, and water. The gas Q is significant because it changes the color of FeSO4 solution to yellow.

### Step 2: Identify the Possible Salts
We need to consider the possible salts that could be reacting with dilute HCl. The options given are:
1. BaS2O3 (Barium Sulfite)
2. Ag2SO3 (Silver Sulfite)
3. AgNO2 (Silver Nitrite)
4. Pb(NO3)2 (Lead Nitrate)

### Step 3: Analyze Each Salt
- **For BaS2O3**: When reacted with dilute HCl, it produces BaCl2, SO2, and water. SO2 does not change the color of FeSO4 solution, so this option is incorrect.

- **For Ag2SO3**: When reacted with dilute HCl, it produces AgCl (precipitate), SO2, and water. Again, SO2 does not change the color of FeSO4 solution, so this option is also incorrect.

- **For AgNO2**: When reacted with dilute HCl, it produces AgCl (precipitate) and NO2 gas. The NO2 gas can change the color of FeSO4 solution to yellow, as it acts as an oxidizing agent. This option is promising.

- **For Pb(NO3)2**: Lead nitrate does not react with dilute HCl upon warming, so this option is incorrect.

### Step 4: Conclusion
Since the only salt that produces a gas (NO2) which changes the color of FeSO4 solution to yellow is AgNO2, we conclude that the salt M in the reaction is **AgNO2 (Silver Nitrite)**.

### Final Answer
The salt M in the above reaction is **AgNO2 (Silver Nitrite)**.

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