Question

When a white crystalline compound ' S ‘ is warmed with concentrated sulfuric acid it gives a gas which mixes in moist air and forms dense white fumes with ammonia. When ’S' is warmed with sodium hydroxide solution, a gas is obtained which turns moist red litmus blue. Name the compound 'S' and each of the gases evolved.

Answer 1

Reaction of ammonium chloride$\left({\mathrm{NH}}_4\mathrm{Cl}\right)$ and sulphuric acid$\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$:

• Ammonium chloride reacts with sulphuric acid to give ammonium sulphate$\left({\left({\mathrm{NH}}_4\right)}_{2}S{O}_4\right)$ and hydrochloric acid$\left(\mathrm{HCl}\right)$.
• $2{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)$
• This hydrochloride gas reacts with ammonia to give dense white fumes of ammonium chloride.
• ${\mathrm{NH}}_3\left(\mathrm{g}\right)+\mathrm{HCl}\left(\mathrm{g}\right)\to {\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)$

Therefore, compound S is ammonium chloride.

• On warming ammonium chloride with sodium hydroxide, we get ammonia which being basic in nature, turns red litmus blue.
• ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)\stackrel{\text{Heat}}{⟶}\mathrm{NaCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{NH}}_3\left(\mathrm{aq}\right)$

Hence, the name of the gases that evolved are Hydrogen chloride gas ($\mathrm{HCl}$) and Ammonia(${\mathrm{NH}}_3$) . Compound S is Ammonium Chloride (${\mathrm{NH}}_4\mathrm{Cl}$)