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Question #44097

Difficulty: 8/10 | 🟢 Free

The Question

A $1.0\text{ L}$ aqueous buffer solution at $298\text{ K}$ contains $0.20\text{ mol}$ of a weak base $B$ and $0.12\text{ mol}$ of its conjugate acid $BH^+$ ( $pK_a = 11.0$ ). To this, $0.50\text{ mol}$ of benzaldehyde ( $\ce{PhCHO}$ ) is added. The base-catalyzed Cannizzaro reaction proceeds, consuming $\ce{OH^-}$ ions. Assume the reaction effectively ceases due to kinetic inhibition exactly when the weak base $B$ is completely exhausted. Given $pK_a(\ce{PhCOOH}) = 4.0$ and $K_w = 10^{-14}$ , calculate the final pH of the solution at this point. (Assume no volume change and $\log 2 = 0.30$ )

Options

A.
7.40
B.
8.65
C.
5.75
D.
7.50

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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

Salt Hydrolysis Logic|Cannizzaro Mechanism|Buffer Capacity and pH Range

Why This Question Is Tricky

Common Mistakes

This problem tests the intersection of organic reaction stoichiometry (Cannizzaro mechanism) and advanced ionic equilibrium.

Students often overlook that the Cannizzaro reaction stoichiometrically consumes base, which depletes the buffer.

The 'Hidden Variable' is the exact proton condition required when the buffer is exhausted, leaving a non-standard mixture of a weak acid ( $BH^+$ ) and a newly generated weak base ( $\ce{PhCOO^-}$ ).

Solving this requires abandoning the Henderson-Hasselbalch equation and applying fundamental charge/mass balance principles.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Equilibrium - Class 11 Part 1
Concept	Salt Hydrolysis Logic
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free