Question

Choose the ion whose aqueous solution is violet colour:

• A. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-}$
• B. $\left[\mathrm{Fe}{\left(\mathrm{SCN}\right)}_3\right]$
• C. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NOS}\right]}^{4-}$
• D. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$

Answer 1

The correct option is C

${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NOS}\right]}^{4-}$

Explanation for the correct option:

(C). The presence of Sulphur in organic compound is detected by fusing the unknown organic compound with sodium in Lassaigne's extract.

$2\mathrm{Na}+\mathrm{S}\underset{}{\overset{}{\to }}{\mathrm{Na}}_2\mathrm{S}$

The final confirmation of sulfide in formed product is identified by reacting with sodium nitroprusside solution. The inference of violet colour confirms the presence of sulphur.

${\mathrm{S}}^{-2}+{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NO}\right]}^{-2}\underset{}{\overset{}{\to }}{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NOS}\right]}^{-4}$

Explanation for the incorrect option:

A. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-}$is used for the detection of ${\mathrm{Fe}}^{3+}$ions. It gives Prussian blue colour. Hence option (A) is incorrect.

B. Red blood colour is given by a complex $\left[\mathrm{Fe}{\left(\mathrm{SCN}\right)}_3\right]$. It detects the presence of sulfur$\left(\mathrm{S}\right)$ and nitrogen$\left(\mathrm{N}\right)$ simultaneously. Hence option (B) is incorrect.

D. ${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$used for the detection of ${\mathrm{Fe}}^{2+}$ions. It forms a blue precipitate. Hence option (D) is incorrect.

Hence option (C) is correct.