See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Ranking Acids by $K_a$ (Acidity) **Identify the compounds:** - I: $HCOOH$ (formic acid) - II: $HOOC-COOH$ (oxalic acid) - III: $HOOC-CH_2-COOH$ (malonic acid) - IV: $CH_3COOH$ (acetic acid) **Key factors affecting $K_a$:** 1. **Electron-withdrawing groups** increase acidity by stabilizing the conjugate base through inductive effects 2. **Multiple electron-withdrawing groups** (like $C=O$) have cumulative effects 3. **Conjugation/resonance** between electron-withdrawing groups further stabilizes the anion **Analysis:** | Acid | Structure | $K_a$ reasoning | |------|-----------|-----------------| | IV (Acetic acid) | $CH_3COOH$ | Weakest—only one carboxyl group, alkyl group donates electrons | | I (Formic acid) | $HCOOH$ | Stronger than IV—H is less electron-donating than $CH_3$, but only one $COOH$ | | III (Malonic acid) | $HOOC-CH_2-COOH$ | Stronger—two electron-withdrawing $COOH$ groups; stabilizes conjugate base | | II (Oxalic acid) | $HOOC-COOH$ | Strongest—two directly adjacent $COOH$ groups with maximum inductive AND resonance stabilization of conjugate base | **Order: $K_a$ (IV) < $K_a$ (I) < $K_a$ (III) < $K_a$ (II)** **Answer: (A) IV < I < III < II** ✓