Question

In qualitative analysis, in III group $N{H}_{4}Cl$ is added before $N{H}_{4}OH$ because

• A. to increase the concentration of $N{H}_4^{+}$ ions
• B. to increase the concentration of $C{l}^{-}$ ions
• C. to reduce the concentration of $O{H}^{-}$ ions
• D. to increase the concentration of $O{H}^{-}$ ions

Answer 1

Answer: (C)

In the presence of strong electrolyte, degree of dissociation of weak electrolyte (having a common ion) gets suppressed due to common ion effect.

$N{H}_{4}Cl$, being a strong electrolyte, dissociates as

$N{H}_{4}Cl⟶N{H}_4^{+}+C{l}^{-}$

and $N{H}_{4}OH$ as,

$\underset{\text{ weak electrolyte }}{N{H}_{4}OH}\rightleftharpoons \underset{\text{ common ion }}{N{H}_4^{+}}+O{H}^{-}$

Due to the presence of common ion, the degree of dissociation of $N{H}_{4}OH$ decreases. Thus, less $O{H}^{-}$ ions are produced which are sufficient only for the precipitation of hydroxides of III group radicals and not of higher group radicals.