Q. In qualitative analysis, in III group $NH_{4}Cl$ is added before $NH_{4}OH$ because
A) To increase the concentration of $NH_{4}^{+}$ ions
B) To increase the concentration of $Cl^{-}$ ions
C) To reduce the concentration of $OH^{-}$ ions
D) To increase the concentration of $OH^{-}$ ions
Explanation
(c)
$NH_{4}Cl$ being a strong electrolyte, dissociates as
$NH_{4}Cl\ \rightarrow {NH}_{4}^{+} + {Cl}^{-}$
and $NH_{4}OH$ as,
$NH_{4}OH\ \ \ \rightleftharpoons \ \ {NH}_{4}^{+}\ \ \ + {OH}^{-}$
weak electrolyte common ion
Due to the presence of common ion, the degree of dissociation of $NH_{4}OH$ decreases. Thus, less ${OH}^{-}$ ions are produced which are sufficient only for the precipitation of hydroxides if III group radicals and not of higher group radicals.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.