AQA A-Level Chemistry Paper 3, 2019: Question 30

✅ Correct Answer

C — guanine & cytosine, 3 hydrogen bonds

💡 Key Knowledge

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
• Hydrogen bonding occurs between lone pairs on highly electronegative atoms (O and N) and partially positive hydrogen atoms bonded directly to nitrogen (N–H).

🧠 Exam Technique & Elimination

Always solve this in two quick elimination steps:

1. Check base pairing rules first:
   • A pairs only with T (eliminates row A: adenine – guanine).
   • G pairs only with C (eliminates row B: cytosine – thymine).
2. Check the number of hydrogen bonds:
   • Row C: guanine – cytosine has 3 bonds → Correct .
   • Row D: adenine – thymine has 2 bonds, but the table says 3 → Incorrect .

❌ Common Errors & Misconceptions

• Confusing base pairs: Pairing purine with purine (A with G) or pyrimidine with pyrimidine (C with T).
• Swapping the bond numbers: Incorrectly thinking A–T has 3 bonds and G–C has 2 bonds.
• Not checking the Data Booklet: If you forget, your Data Booklet provides the molecular structures of all four bases. You can line them up to see where the N–H ··· O=C and N–H ··· N hydrogen bonds form!