Question

An orange solid $A$ on heating gives a green residue $B$, a colourless gas $C$ and water vapours. The dry gas $C$ upon pasing over heated Mg gave a white solid D which upon subsequent reaction with water gave a gas $E$ that gave dense white fumes with HCl. Identify $D$

• A. $Fe(NH_3)Cl_2$
• B. $CuN_2$
• C. $Mg_3N_2$
• D. $NH_4Cl$
• E. $FeCl_2$

Answer 1

Answer: (C)

An orange solid, i.e. $( NH _{4})_{2} Cr _{2} O _{7}(A)$ on heating gives a green residue, i.e. $Cr _{2} O _{3}(B)$, a colourless gas, i.e. $N _{2}(C)$ and water vapours. The dry gas $( N _{2})$ upon passing over heated Mg gave a white solid, i.e. $Mg _{3} N _{2}(D)$ which upon subsequent reaction with water gave a gas, i.e. $NH _{3}(E)$ that gave dense white fumes with HCl.

Reactions involved are

$\underset{(A)}{( NH _{4})_{2} Cr _{2} O _{7}} \longrightarrow \underset{(C)}{N _{2}} \uparrow+ \underset{(B)}{Cr _{2} O _{3}}+4 H _{2} O$

$\underset{(C)}{N _{2}}+3 Mg \longrightarrow \underset{(D)}{Mg _{3} N _{2}}$

$\underset{(D)}{Mg _{3} N _{2}}+ H _{2} O \longrightarrow Mg ( OH )_{2}+ \underset{(E)}{NH _{3}}$

$\underset{(E)}{NH _{3}}+ dilHCl \longrightarrow \underset{\text{dense white fumes }}{NH _{4} Cl}$