NEET Chemistry: Acidic strength and factors affecting it Practice Questions
14 free multiple-choice questions on Acidic strength and factors affecting it (Carboxylic Acids) for NEET Chemistry, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.
Practice thousands more NEET questions in the app →Q1. The acidity of a carboxylic acid is generally higher when the C-O bond is conjugated with a π-system. This effect is due to:
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Correct answer: A — Resonance stabilization
When the C-O bond is conjugated with a π-system (like a benzene ring), the negative charge can be delocalized over a larger system via resonance, significantly increasing the stability of the conjugate base.
Q2. The acidity of a carboxylic acid is primarily influenced by the stability of the resulting carboxylate anion. Stability is increased when the negative charge is delocalized. Which statement correctly describes the primary factor affecting the acidity of a carboxylic acid?
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Correct answer: B — The extent of conjugation with π-system
Acidity is enhanced when the resulting carboxylate anion is stabilized. Delocalization of the negative charge through resonance (conjugation with π-systems) is the primary mechanism for stabilizing the anion and increasing acidity.
Q3. Consider the following series: CH₃COOH, C₂H₅COOH, C₃H₇COOH, C₄H₉COOH. Which one is the strongest acid?
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Correct answer: C — C₄H₉COOH
For straight-chain carboxylic acids, acidity increases with the increase in the size of the alkyl group (and thus the chain length) because the carboxylate ion is better stabilized by dispersion forces as the chain length increases.
Q4. Which of the following carboxylic acids is the strongest acid?
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Correct answer: C — Acetic acid
Acidity in a homologous series (like straight-chain carboxylic acids) increases with the increasing chain length due to the increased stability of the carboxylate ion (less steric hindrance and less electron-withdrawing effect of the chain).
Q5. Which of the following compounds is generally expected to be the strongest carboxylic acid?
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Correct answer: C — Hexanoic acid
Acidity in saturated aliphatic carboxylic acids generally increases with the increasing length of the carbon chain due to the increased stability of the resulting carboxylate ion (less steric hindrance and better hyperconjugation).
Q6. Consider the effect of an electron-withdrawing group (EWG) placed at the α-carbon of a carboxylic acid. How does the acidity typically change?
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Correct answer: D — Acidity increases
An EWG stabilizes the carboxylate anion by delocalizing the negative charge. This stabilization makes the acid stronger (more acidic).
Q7. Acidity of carboxylic acids is strongly influenced by the stability of the conjugate base. Which structural feature best contributes to the stability of the carboxylate ion?
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Correct answer: D — Presence of an electron-withdrawing group (EWG)
Electron-withdrawing groups (EWGs) pull electron density away from the C-O bond, stabilizing the resulting negative charge on the carboxylate anion, thus increasing acidity.
Q8. Which effect causes the greatest decrease in the pKₐ of a carboxylic acid?
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Correct answer: D — Resonance effect of -NO₂
Strongest decrease in pKₐ occurs when the substituent exerts a strong electron-withdrawing inductive effect, which directly stabilizes the conjugate base. -NO₂ is a strong EWG.
Q9. Which factor primarily increases the acidity of a carboxylic acid?
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Correct answer: A — Introducing an electron-withdrawing group (EWG) in the α-position
Electron-withdrawing groups (EWGs) stabilize the conjugate base (carboxylate ion) by dispersing the negative charge, thereby increasing the acidity.
Q10. Which substituent will increase the acidity of a carboxylic acid?
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Correct answer: C — -NO₂
Electron-withdrawing groups (EWG), such as -NO₂, pull electron density away from the carboxyl group, stabilizing the resulting carboxylate anion and increasing the acid's strength.
Q11. Which factor primarily increases the stability of the carboxylate anion, thereby increasing the acidity of the carboxylic acid?
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Correct answer: A — Resonance effect
The stability of the carboxylate anion (RCOO⁻) is increased by delocalization of the negative charge through resonance involving the adjacent oxygen atoms. This resonance effect is the primary reason for increased acidity.
Q12. If the acidity of a carboxylic acid is compared based on the electronic environment, which structure will have the most acidic proton: CH₃COOH or CF₃COOH?
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Correct answer: D — CF₃COOH
The -CF₃ group is strongly electron-withdrawing via induction, making the COOH group more electrophilic (more proton-withdrawing) and thus increasing the acidity of CF₃COOH compared to CH₃COOH.
Q13. If a substituent is attached to the α-carbon, the effect on the acidity is primarily governed by:
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Correct answer: A — The ability of the substituent to withdraw or donate electrons
The electronic effect (inductive and resonance) of the substituent on the carboxyl group dictates the stability of the anion and thus the acid strength.
Q14. The acidity of propanoic acid (CH₃CH₂COOH) compared to acetic acid (CH₃COOH) is primarily determined by which factor?
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Correct answer: B — Inductive effect
Propanoic acid has an extra CH₂ group, which increases the electron-donating inductive effect, slightly decreasing acidity compared to the less substituted acetic acid.
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