The correct order of decreasing acidity of the following aliphatic acids is

The correct order of decreasing acidity is:

HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH

Acidity of carboxylic acids is governed mainly by the stability of their conjugate bases. All four acids have the same carboxylate anion core (‑COO⁻) after deprotonation, so differences arise from how the attached alkyl groups influence that anion.

1. Formic acid (HCOOH) has no alkyl substituent attached to the carbonyl carbon. The conjugate base (formate) is not destabilised by electron‑donating groups, making it the most acidic of the series.

2. Acetic acid (CH₃COOH) contains a single methyl group. Methyl is weakly electron‑releasing (+I effect), which slightly destabilises the acetate anion compared with formate, thus lowering its acidity.

3. 2‑Methylpropanoic acid ((CH₃)₂CHCOOH) has two alkyl groups attached to the α‑carbon. The combined inductive (+I) effect of these groups further reduces the stability of the conjugate base, making it less acidic than acetic acid.

4. Pivalic acid ((CH₃)₃CCOOH) possesses three methyl groups (tert‑butyl). The strongest overall electron‑donating inductive effect among the four compounds most destabilises the conjugate base, giving it the lowest acidity.

Typical pK_a values illustrate this trend:

HCOOH ≈ 3.75
CH₃COOH ≈ 4.76
(CH₃)₂CHCOOH ≈ 5.0
(CH₃)₃CCOOH ≈ 5.5

Higher pK_a means weaker acidity, corroborating the order listed above.

Therefore, the decreasing acidity sequence is exactly as stated: HCOOH > CH₃COOH > (CH₃)₂CHCOOH > (CH₃)₃CCOOH.