2025 O Level Chem P1 Q11 — Stoichiometry / Mole Calculations Solution

Answer: D. Read the question precisely: 'the total number of electrons shared by the covalent bonds'. Count individual electrons, not shared pairs.

Combustion: \(\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}\). Moles of \(\text{CO}_2\) = \(0.50 \times 3 = 1.50\) mol.

Each \(\text{CO}_2\) molecule has structure O=C=O — two C=O double bonds. Each double bond contains 4 shared electrons (2 shared pairs × 2 electrons each). So one \(\text{CO}_2\) has \(2 \times 4 = 8\) shared electrons.

Total electrons shared = \(1.50 \times 6.02 \times 10^{23} \times 8 = 7.22 \times 10^{24}\) → D.

Option C (\(3.6 \times 10^{24}\)) is the count of shared pairs, not individual electrons — a trap.