Which of the following is the most acidic?

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Accepted Answer

Step-by-step reasoning:

1. Identify the acids:
   - (1) HCOOH = formic acid
   - (2) C2H5COOH = propionic acid
   - (3) C3H7COOH = butanoic (butyric) acid
   - (4) CH3COOH = acetic acid

2. Acidity of carboxylic acids depends on the stability of the conjugate base (carboxylate anion). Electron-withdrawing groups stabilize the negative charge and increase acidity; electron-donating (alkyl) groups destabilize it and decrease acidity.

3. Comparison:
   - Formic acid has no alkyl group (only H), so its conjugate base is least destabilized by electron-donation.
   - Acetic, propionic and butanoic acids have increasing alkyl character, which are electron-releasing and slightly reduce acidity.

4. pKa values (for confirmation): formic acid $\mathrm{p}K_a\approx 3.75$, acetic $\mathrm{p}K_a\approx 4.76$, propionic and butanoic around $4.8$–$4.9$. Lower $\mathrm{p}K_a$ = stronger acid.

Conclusion: Formic acid (HCOOH) is the most acidic.

FINAL_ANSWER: a

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