See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Identifying Stronger Acids than Acetic Acid **Step 1: Recall acetic acid's $K_a$ value** Acetic acid ($CH_3COOH$) has $K_a \approx 1.8 \times 10^{-5}$, making it a weak acid. **Step 2: Analyze each option by acid strength** | Acid | Structure | $K_a$ | Strength | |------|-----------|-------|----------| | Formic | $HCOOH$ | $1.8 \times 10^{-4}$ | **Stronger** ✓ | | Acetic | $CH_3COOH$ | $1.8 \times 10^{-5}$ | Reference | | Propionic | $C_2H_5COOH$ | $1.3 \times 10^{-5}$ | Weaker | | Butyric | $C_3H_7COOH$ | $1.5 \times 10^{-5}$ | Weaker | | Isobutyric | $(CH_3)_2CHCOOH$ | $1.2 \times 10^{-5}$ | Weaker | **Step 3: Explain the trend** Among carboxylic acids, **formic acid is anomalously stronger** because it has the smallest alkyl group (H instead of $CH_3$). The electron-withdrawing effect from the H-atom (lack of electron-donating alkyl groups) stabilizes the conjugate base $HCOO^-$, increasing acid strength. Longer-chain carboxylic acids become progressively weaker due to electron-donating inductive effects from extended alkyl chains. **Answer: (2) Formic acid** is stronger than acetic acid.