See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Identifying Minimum pKₐ **Understanding pKₐ:** Minimum $pK_a$ means maximum acid strength (since $pK_a = -\log K_a$). A stronger acid has larger $K_a$ and smaller $pK_a$. **Analyzing each compound:** **(1) $C_6H_5OH$ (Phenol):** $pK_a \approx 10$ - Aromatic OH; conjugate base (phenoxide) stabilized by resonance through the benzene ring - More acidic than aliphatic alcohols **(2) $HCOOH$ (Formic acid):** $pK_a \approx 3.75$ - Carboxylic acid; conjugate base stabilized by O-O resonance and inductive effects - **Much stronger acid than phenol** **(3) $C_2H_5OH$ (Ethanol):** $pK_a \approx 15-16$ - Aliphatic alcohol; no resonance stabilization - Weakest acid in this series **(4) $CH_3C \equiv CH$ (Propyne):** $pK_a \approx 25$ - Terminal alkyne; very weak acid - Weakest among all options **Conclusion:** Formic acid ($HCOOH$) has the **lowest $pK_a$ ≈ 3.75**, making it the strongest acid. The carboxyl group's resonance-stabilized conjugate base provides exceptional acid strength. **Answer: (2) $HCOOH$** ✓