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JEE Main 20244 AprilMorning Shift
Exam
JEE Main
Year
2024
Date
4 April
Shift
Morning Shift
Subject
Chemistry
Chapter
Practical Chemistry

JEE Main 2024 · 4 April · Morning Shift · Chemistry · Practical Chemistry

In the precipitation of the iron group (III) in qualitative...

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

Options

  1. Aincrease concentration of (\mathrm{Cl}^{-})ions
  2. Bincrease concentration of (\mathrm{NH}_4{ }^{+})ions
  3. Cprevent interference by phosphate ions
  4. Ddecrease concentration of ({ }^{-} \mathrm{OH}) ions

Correct answer

D. decrease concentration of ({ }^{-} \mathrm{OH}) ions

Step-by-step solution

NH4OH⇌NH4++OH−NH4Cl→NH4++Cl\begin{aligned} & \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} \\ & \mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{NH}_4^{+}+\mathrm{Cl} \end{aligned}
Due to common ion effect of NH4+\mathrm{NH}_4^{+}, [OH−]\left[\mathrm{OH}^{-}\right]decreases in such extent that only group-III cation can be precipitated, due to their very low Ksp\mathrm{K}_{\mathrm{sp}} in the range of 10−3810^{-38}.
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