SSC CGL 2024 Tier-I · 17 September 2024, Shift 3 · General Awareness · Question 7 of 100

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Which of the following is the most acidic?

(a)C₃H₇COOH
(b)CH₃COOH
(c)C₂H₅COOH
(d)HCOOH

Answer

Why

Correct — D. HCOOH, formic (methanoic) acid, is the strongest acid of the four — option (d).

All four are carboxylic acids, R-COOH. What differs is R. Formic acid has only a hydrogen there. The other three carry an alkyl group — CH₃, C₂H₅, C₃H₇.

Alkyl groups donate electrons. That destabilises the carboxylate ion left when the proton leaves, and weakens the acid.

The pKa values agree: formic acid 3.75, the other three about 4.8. Lower pKa means stronger acid.

Why the others are wrong

(a)
C₃H₇COOH — C₃H₇COOH is butyric (butanoic) acid in its straight-chain form, pKa about 4.82. Its three-carbon alkyl group donates electrons and leaves it far weaker than formic acid.
(b)
CH₃COOH — CH₃COOH is acetic acid, pKa about 4.76. One methyl group is enough to make it roughly ten times weaker than formic acid.
(c)
C₂H₅COOH — C₂H₅COOH is propionic (propanoic) acid, pKa about 4.88. Its ethyl group donates electrons, so it too sits well below formic acid.

Concept

How strong a carboxylic acid is depends on how stable its conjugate base, the carboxylate ion R-COO⁻, is.

NCERT Class 12 states the rule. Electron-withdrawing groups stabilise the carboxylate and strengthen the acid. Electron-donating groups destabilise it and weaken the acid.

Alkyl groups donate electrons, so putting any alkyl group in place of formic acid's hydrogen lowers the acidity. Chlorine withdraws electrons, so chloroacetic acid is stronger than acetic acid.

Acid strength is compared through pKa: the smaller it is, the stronger the acid.

The three alkyl acids do not fall in a neat chain-length order. Measured pKa values are acetic 4.76, propanoic 4.88 and butanoic 4.82, so butanoic is marginally the stronger of the last two.

The question needs only the most acidic, and that is settled by formic acid having no alkyl group at all.

Key facts

Formic acid, HCOOH, is the simplest carboxylic acid, with a pKa of about 3.75.
Acetic acid, CH₃COOH, has a pKa of about 4.76, so formic acid is roughly ten times stronger.
Electron-donating alkyl groups weaken a carboxylic acid by destabilising its carboxylate ion.
Electron-withdrawing groups such as halogens strengthen a carboxylic acid.

Study next

Inductive effect and the acidity of carboxylic acidsEffect of halogen substitution on acidity — chloroacetic and trifluoroacetic acidsCommon and IUPAC names of carboxylic acids, from formic to butyric

Common traps

Assuming a bigger molecule makes a stronger acid, when adding an alkyl group here weakens it.
Ranking the alkyl acids strictly by chain length, when propanoic and butanoic acids are almost equal.

Carboxylic acids are also asked by formula and name. Butyric acid, the C₃H₇COOH of option (a), is identified from CH₃CH₂CH₂CO₂H at 09 Sep 2024, 16:00, GA Q.4, and acetic acid as the souring agent added to vinegar and pickles at 25 Sep 2024, 16:00, GA Q.9.

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 152 SSC CGL questions on it, every solved shift.