Question

In qualitative analysis, in III group $ NH_4Cl $ is added before $ NH_4OH $ 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

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.

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

$NH _{4} Cl \longrightarrow NH _{4}^{+}+ Cl ^{-}$

and $NH _{4} OH$ as,

$\underset{\text { weak electrolyte }}{ NH _{4} OH } \rightleftharpoons \underset{\text { common ion }}{ NH _{4}^{+}}+ OH ^{-}$

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 of III group radicals and not of higher group radicals.