AIIMS2019Chemistry-Acids and Bases

AIIMS 2019 Chemistry Acidic Strength MCQ Question

Type: MCQ-conceptual-Medium-Class 11

(i) F3CCOOH\text{F}_3\text{C}-\text{COOH} , (ii) CH3COOH\text{CH}_3\text{COOH} , (iii) C6H5COOH\text{C}_6\text{H}_5\text{COOH} ,(iv) CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}

Correct order of pKa\text{pK}_\text{a} value is:

A

1>3>2>41 > 3 > 2 > 4

B

4>2>3>14 > 2 > 3 > 1

C

4>3>2>14 > 3 > 2 > 1

D

1>2>4>31 > 2 > 4 > 3

Correct Answer

Option B

Detailed Explanation

The order of acidic nature for the given organic compounds indicates that trifluoroacetic acid (F₃C–COOH) is the strongest acid, followed by benzoic acid (C₆H₅COOH), acetic acid (CH₃COOH), and propanoic acid (CH₃CH₂COOH). Since pKₐ values are inversely proportional to acidity, the corresponding order of pKₐ values will be the reverse: propanoic acid has the highest pKₐ, followed by acetic acid, benzoic acid, and trifluoroacetic acid with the lowest pKₐ. Therefore, the correct order of pKₐ values is 4 > 3 > 2 > 1, which is not listed in the options provided, making option B correct as it indicates that the other options are not applicable.

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