Consider the following reactions and answer the following questions.
M(Double salt)+`NH_(4)Cl(s)+NH_(4)OH to `No ppt.
M(double salt)+`NaOH` solution`toN uarr+Pdarr` (coloured ppt.)
Q. `Pdarr+`conc. `HCl to Q`(coloured solution)
Incorrect statement about Q is:

Options

A. It can exist in dimeric form
B. Its aqueous solution is acidic
C. It is used in methylene blue test for `H_(2)S`
D. On passing `Cl_(2)` gas colour of aqueous solution of Q changes

Answer

To solve the question regarding the reactions involving the double salt M and to identify the incorrect statement about Q, we will analyze each step of the reactions provided.

### Step-by-Step Solution:

1. **Identify the Double Salt (M)**:
- M is identified as Mohr's salt, which has the formula \( \text{FeSO}_4 \cdot \text{(NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O} \).

**Hint**: Recognize that Mohr's salt is a common double salt containing iron and ammonium.

2. **First Reaction**:
- The reaction of Mohr's salt (M) with \( \text{NH}_4\text{Cl} \) and \( \text{NH}_4\text{OH} \) results in no precipitate. This indicates that the ions present do not form any insoluble compounds under these conditions.

**Hint**: Consider the solubility rules for ammonium salts and hydroxides.

3. **Second Reaction**:
- The second reaction involves Mohr's salt reacting with \( \text{NaOH} \). This leads to the liberation of nitrogen gas (N) and the formation of a colored precipitate (P), which is \( \text{Fe(OH)}_2 \) (ferrous hydroxide).

**Hint**: Recall that ferrous hydroxide is typically a green precipitate.

4. **Formation of Q**:
- The precipitate \( P \) (ferrous hydroxide) reacts with concentrated \( \text{HCl} \) to form \( \text{FeCl}_2 \) (Q), which is a colored solution. The color of \( \text{FeCl}_2 \) is green.

**Hint**: Understand that the reaction of a metal hydroxide with an acid typically results in the formation of a salt and water.

5. **Properties of Q**:
- Now we analyze the properties of \( \text{FeCl}_2 \):
- It exists in dimeric form in the vapor phase.
- Its aqueous solution is acidic.
- It is not used in the methylene blue test for \( \text{H}_2\text{S} \); instead, \( \text{FeCl}_3 \) is used for that test.
- On passing chlorine gas through \( \text{FeCl}_2 \), it changes to \( \text{FeCl}_3 \), which is yellow.

**Hint**: Be aware of the specific tests and reactions involving iron(II) and iron(III) compounds.

6. **Identify the Incorrect Statement**:
- The incorrect statement about \( Q \) is that it is used in the methylene blue test for \( \text{H}_2\text{S} \). This is false because \( \text{FeCl}_3 \) is the compound used in that test, not \( \text{FeCl}_2 \).

### Conclusion:
The incorrect statement about Q is that it is used in the methylene blue test for \( \text{H}_2\text{S} \).