Solution

Moles of $H_2=\dfrac{1.0}{2}=0.5$, so number of molecules $=0.5N_A$. Checking option (a): moles of $N_2$ in 14 g $=\dfrac{14}{28}=0.5$, giving $0.5N_A$ molecules — same as $H_2$. ($H_2O$ (18 g) gives 1 mol $=N_A$ molecules; CO (16 g) gives $\dfrac{4}{7}$ mol; $N_2$ (28 g) gives 1 mol — neither matches $0.5N_A$.)

Concept Mole concept and Avogadro's number

Some Basic Concepts of Chemistry in past papers

49 questions from this chapter have appeared across 23 exam years.