AQA A-Level Chemistry Paper 3, 2019: Question 35

✅ Correct Answer

Option D

At pH = 14 :

• Both carboxylic acid groups lose a proton to form carboxylate ions:
  -COOH → -COO⁻ + H⁺
• The amino group remains neutral (deprotonated) because of high [OH⁻]:
  -NH₂
• Overall structure has a net charge of -2.

💡 Key Knowledge: Amino Acids vs pH

• Low pH (Acidic, excess H⁺): Amino groups are protonated to -NH₃⁺ ; carboxylic acid groups stay as -COOH . Net positive charge.
• Isoelectric point / neutral pH: Forms a zwitterion: -COO⁻ and -NH₃⁺ .
• High pH (Alkaline, excess OH⁻): Carboxylic acid groups lose H⁺ to become -COO⁻ ; amine groups remain unprotonated as -NH₂ . Net negative charge.

🧠 Exam Technique & Option Elimination

• Step 1: Inspect the pH. A pH = 14 indicates an extremely concentrated alkaline environment (e.g. 1.0 mol dm⁻³ NaOH).
• Step 2: Check the amine group. Under strongly basic conditions, an amine cannot accept a proton, so it must be -NH₂ . This immediately eliminates B and C (both show -NH₃⁺ ).
• Step 3: Check carboxylic acid groups. Under basic conditions, -COOH groups act as acids and donate protons to OH⁻ to form -COO⁻ and H₂O. Aspartic acid has two carboxylic acid groups (one on the main chain and one on the side chain). Both must be deprotonated into -COO⁻ . This eliminates A.
• Conclusion: Only D correctly shows both groups as -COO⁻ and the amine as -NH₂ .

❌ Common Misconceptions & Traps

• Forgetting the side-chain group: Students often deprotonate the main-chain -COOH but forget that aspartic acid has a second carboxylic acid in its -CH₂COOH side chain.
• Confusing high and low pH: Mixing up which group is charged in acid vs alkali. Remember: Base removes protons (leaves negative -COO⁻ ), Acid adds protons (creates positive -NH₃⁺ ).
• Selecting the zwitterion (Option C): Zwitterions only predominate around the amino acid's specific isoelectric point ( pH ~ 3 for aspartic acid), never at pH = 14 .