A metal M and its compound can give the following observable changes in a consequence of reactions
`Munderset(HNO_(3))overset("dilute")(rarr)["Colourless Solutions"]underset(NaOH)overset("aqueous")(rarr)["White Precipitate"]underset("NaOH(aq)")overset("excess")(rarr)["Colourless Solutions"]overset(H_(2)S)(rarr)["White Precipitate"]`

Options

A. `Mg`
B. `Pb`
C. `Zn`
D. `Sn`

Answer

To solve the problem step by step, we will analyze the given reactions and identify the metal M based on the observable changes mentioned.

### Step 1: Reaction with Dilute HNO3
The metal M reacts with dilute nitric acid (HNO3) to form a colorless solution. This indicates that the metal M forms a soluble nitrate compound.

**Chemical Reaction:**
\[ M + 2HNO_3 \rightarrow M(NO_3)_2 + H_2 \]

### Step 2: Reaction with Aqueous NaOH
The colorless solution formed in the first step is then treated with aqueous sodium hydroxide (NaOH), resulting in the formation of a white precipitate. This suggests that the metal hydroxide formed is insoluble in water.

**Chemical Reaction:**
\[ M(NO_3)_2 + 2NaOH \rightarrow M(OH)_2 \downarrow + 2NaNO_3 \]

Here, \( M(OH)_2 \) is the white precipitate.

### Step 3: Reaction with Excess NaOH
When the white precipitate is treated with excess NaOH, it dissolves to form a colorless solution. This indicates that the metal hydroxide can react with excess NaOH to form a soluble complex.

**Chemical Reaction:**
\[ M(OH)_2 + 2NaOH \rightarrow Na_2M(OH)_4 + 2H_2O \]

### Step 4: Reaction with H2S
Finally, the colorless solution obtained from the previous step is treated with hydrogen sulfide (H2S), resulting in the formation of a white precipitate. This suggests that the metal forms a sulfide that is insoluble in water.

**Chemical Reaction:**
\[ Na_2M(OH)_4 + H_2S \rightarrow MS \downarrow + 2NaOH + 2H_2O \]

Here, \( MS \) is the white precipitate formed.

### Conclusion
Based on the reactions, we can deduce that the metal M is Zinc (Zn). The sequence of reactions aligns with the behavior of zinc when reacting with dilute HNO3, NaOH, and H2S.

### Final Answer
The metal M is **Zinc (Zn)**.

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