2023 O Level Chem P1 Q14 — The Mole Solution

Answer: C. The number of particles is proportional to the number of moles (\(n \times N_A\)); comparing samples reduces to comparing moles. Calculate \(n = \dfrac{\text{mass}}{M_r}\) for each:

A: 2 g of \(\text{H}_2\), \(M_r = 2\) → \(n = 2/2 = 1.00\) mol.
B: 4 g of He, \(A_r = 4\) → \(n = 4/4 = 1.00\) mol.
C: 8 g of \(\text{O}_2\), \(M_r = 32\) → \(n = 8/32 = 0.25\) mol.
D: 22 g of \(\text{CO}_2\), \(M_r = 44\) → \(n = 22/44 = 0.50\) mol.

The smallest number of moles is C (0.25 mol), so C has the smallest number of particles. Note we are comparing moles of particles as defined in each option (He atoms vs \(\text{H}_2\) molecules etc.) — the question asks for total particles of that type, not atoms.