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Question #44320

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The Question

Freshly precipitated $\ce{BeO}$ and $\ce{Al2O3}$ exhibit a diagonal relationship, both dissolving completely in $1.0\text{ M } \ce{NaOH(aq)}$ at $298\text{ K}$ to form soluble mononuclear hydroxo-complexes. When solid $\ce{NaOH}$ is continuously added to this equimolar saturated solution, the concentrations of $\ce{Na+}$ and $\ce{OH-}$ increase drastically. Governed by the common ion effect and the lattice energy of the respective complex salts, one species selectively crystallizes out as a white precipitate. Identify the chemical formula of this precipitate.

Options

A.
$\ce{Na2[Be(OH)4]}$
B.
$\ce{Na[Al(OH)4]}$
C.
$\ce{Be(OH)2}$
D.
$\ce{Al(OH)3}$

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Acid-Base Nature of Oxides|Diagonal Relationship|Lattice Energy Factors

Why This Question Is Tricky

Common Mistakes

The question tests a 'Chemical Exception' where students blindly apply the diagonal relationship to assume identical solubility behavior for Be and Al in all basic conditions.

The hidden variable is the stoichiometry of the complex salts ( $\ce{Na2[Be(OH)4]}$ vs $\ce{Na[Al(OH)4]}$ ).

Students often assume excess $\ce{NaOH}$ guarantees infinite solubility for amphoteric oxides.

They overlook that adding excess reagent introduces a massive Common Ion Effect from $\ce{Na+}$ , which disproportionately forces the precipitation of the beryllate due to its second-order dependence on $[\ce{Na+}]$ , overriding the expected amphoteric trend.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Acid-Base Nature of Oxides
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free