Chemistry · Practical Inorganic chemistry (Qualitative Analysis)

JEE Main 2024 — 4 April, Shift 1 — Question 62

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :

Option A:
prevent interference by phosphate ions
Option B:
decrease concentration of ${ }^{-} \mathrm{OH}^{-}$ ions
Correct
Option C:
increase concentration of $\mathrm{Cl}^{-}$ ions
Option D:
increase concentration of $\mathrm{NH}_{4}{ }^{+}$ ions

Answer: B

Step-by-step solution

$\mathrm{NH}_{4} \mathrm{OH} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{OH}^{-}$ $\mathrm{NH}_{4} \mathrm{Cl} \rightarrow \mathrm{NH}_{4}^{+}+\mathrm{Cl}$

Due to common ion effect of $\mathrm{NH}_{4}^{+}$ , $\left[\mathrm{OH}^{-}\right]$ decreases in such extent that only group-III cation can be precipitated,

due to their very low $\mathrm{K}_{\mathrm{sp}}$ in the range of $10^{-38}$ .

Answer key and solution verified before publishing.

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Exam: JEE Main 2024
Paper: 4 April, Shift 1
Subject: Chemistry
Chapter: Practical Inorganic chemistry (Qualitative Analysis)
Topic: Identification of Cations (Groups 1 to 3)