See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The pKa value reflects the acid strength of a compound — lower pKa means stronger acid (more willing to donate a proton). Step 1: Assess the acidity of picric acid (2,4,6-trinitrophenol). Picric acid has three strongly electron-withdrawing nitro groups (-NO2) at the ortho and para positions of the phenol ring. These groups stabilize the phenoxide anion enormously through resonance and inductive effects. This makes picric acid a very strong acid with pKa ≈ 0.4. Step 2: Assess the acidity of acetic acid (CH3COOH). Acetic acid is a carboxylic acid. The conjugate base (acetate) is stabilized by resonance delocalization over the two oxygen atoms. Its pKa ≈ 4.75, making it a moderately weak acid — stronger than phenol but much weaker than picric acid. Step 3: Assess the acidity of phenol (C6H5OH). Phenol is a weak acid. The phenoxide anion is stabilized by resonance with the aromatic ring, but without electron-withdrawing substituents this stabilization is limited. Its pKa ≈ 10.0, making it the weakest acid of the three. Step 4: Order of pKa values (lowest to highest = strongest to weakest acid): Picric acid (0.4) < Acetic acid (4.75) < Phenol (10.0) Step 5: Match to options. Option (a) states: Picric acid 0.4, acetic acid 4.75, phenol 10.0 — this matches exactly. Why other options fail: - (b) assigns 0.4 to acetic acid, which is incorrect; acetic acid is not nearly that acidic. - (c) assigns 4.75 to phenol and 10.0 to acetic acid, reversing their actual values. - (d) assigns 0.4 to phenol, which is far too acidic for unsubstituted phenol. Therefore, the correct answer is A.