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Question #45069

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The Question

Consider a sparingly soluble neutral amino acid whose cationic form is \ce{H2A+}. The acid dissociation constants are $pK_{a1} = 2.00$ and $pK_{a2} = 5.00$ at $298\text{ K}$ . The intrinsic solubility of the neutral zwitterion \ce{HA_{(s)}} in pure water is $S_0 = 0.10\text{ mol/L}$ .

To $1.0\text{ L}$ of a saturated solution of \ce{HA} (which contains excess undissolved solid \ce{HA}), $0.10\text{ moles}$ of its sodium salt, \ce{NaA}, is added. Assuming no volume change upon addition, which of the following statements correctly describes the final equilibrium state of the solution?

Options

A.
The pH is $5.00$ and additional solid \ce{HA} precipitates.
B.
The pH is $5.00$ and more solid \ce{HA} dissolves.
C.
The pH is $3.50$ and the mass of solid \ce{HA} remains unchanged.
D.
The pH is $2.00$ and additional solid \ce{HA} precipitates.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Acidity and Basicity (pKa/pKb)|Amphiprotic Ion pH|Common Ion Effect on Solubility

Why This Question Is Tricky

Common Mistakes

This problem tests the intersection of phase equilibria and amphiprotic buffer systems.

Students often mistakenly assume that adding a salt of a conjugate base to a zwitterion will either not affect the solid (since it is a different species) or that the pH will remain at the isoelectric point ( $pI = 3.50$ ).

The hidden mechanism is that the intrinsic solubility of the zwitterion anchors the $[\ce{HA}]$ concentration, turning the system into a perfect buffer.

Furthermore, the addition of the common ion (\ce{A-}) suppresses the natural disproportionation of the zwitterion ( $2\ce{HA} \rightleftharpoons \ce{H2A+} + \ce{A-}$ ), which lowers the overall analytical solubility of the amino acid and forces precipitation.

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