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Question #45070

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The Question

An aqueous solution at $298\text{ K}$ contains $0.1\text{ M}$ each of $La^{3+}$ , $Gd^{3+}$ , and $Lu^{3+}$ ions. A dilute solution of $NaOH(aq)$ is added dropwise to this mixture. Due to the imperfect shielding of 4f electrons, the basic character of lanthanide hydroxides varies across the series. Let $pH_{La}$ , $pH_{Gd}$ , and $pH_{Lu}$ be the exact pH values at which the precipitation of $La(OH)_3$ , $Gd(OH)_3$ , and $Lu(OH)_3$ commences, respectively. Assuming no complexation occurs, which of the following represents the correct order of these pH values?

Options

A.
$pH_{Lu} < pH_{Gd} < pH_{La}$
B.
$pH_{La} < pH_{Gd} < pH_{Lu}$
C.
$pH_{Gd} < pH_{La} < pH_{Lu}$
D.
$pH_{La} = pH_{Gd} = pH_{Lu}$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Acidity and Basicity (pKa/pKb)|F-block Basic Character|Periodic Properties of Lanthanides

Why This Question Is Tricky

Common Mistakes

The question tests a critical 'Chemical Exception' regarding the horizontal trend of basicity in the f-block.

Students often fall into the trap of applying vertical s-block trends (where basicity increases down a group due to increasing size and lower ionization energy) or assume that basicity increases with atomic number.

The hidden variable here is the 'Lanthanide Contraction'.

The poor shielding of 4f electrons causes a decrease in ionic radius across the series, which increases the cation's polarizing power.

By Fajans' Rule, this increases the covalent character of the Ln-OH bond, thereby decreasing basicity and Ksp.

The 'Litmus Test' is whether the student recognizes that the least basic hydroxide (Lu(OH)3) will precipitate first at the lowest pH.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Acidity and Basicity (pKa/pKb)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free