Paragraph: p-amino-N, N-dimethylaniline is added to a strongly acidic solution of $X$. The resulting solution is treated with a few drops of aqueous solution of $Y$ to yield blue colouration due to the formation of methylene blue. Treatment of the aqueous solution of $Y$ with the reagent potassium hexacyanoferrate(II) leads to the formation of an intense blue precipitate. The precipitate dissolves an excess addition of the reagent. Similarly, treatment of the solution of $Y$ with the solution of potassium hexacyanoferrate(III) leads to a brown colouration due to the formation of $Z$.Question: The compound $Z$, is

• A

  $\mathrm{Mg}_2\left[\mathrm{Fe}(\mathrm{CN})_6\right]$
• Correct answer
  B

  $\mathrm{Fe}\left[\mathrm{Fe}(\mathrm{CN})_6\right]$
• C

  $\mathrm{Fe}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]_3$
• D

  $\mathrm{K}_2 \mathrm{Zn}_3\left[\mathrm{Fe}(\mathrm{CN})_6\right]_2$

Solution

$p$-amino- $\mathrm{N}$, N-dimethylaniline reacts with sulphide ion $\left(\mathrm{S}^{2-}\right)$ in presence of $\mathrm{Fe}^{3+}$ ion to form blue coloured solution of methylene blue :

Hence, the correct answer is option: 'B'.