Solution

Number of molecules is proportional to the number of moles $= \dfrac{\text{mass}}{\text{molar mass}}$.
7 g $N_2$: $\dfrac{7}{28} = 0.25$ mol
2 g $H_2$: $\dfrac{2}{2} = 1$ mol
16 g $NO_2$: $\dfrac{16}{46} = 0.35$ mol
16 g $O_2$: $\dfrac{16}{32} = 0.5$ mol
So 2 g of $H_2$ (1 mol, $6.022 \times 10^{23}$ molecules) has the maximum number of molecules.

Concept Comparing number of moles

Mole Concept in past papers

9 questions from this chapter have appeared across 7 exam years.