Which of the following contains maximum number of molecules?

When the mass is the same, the number of molecules is inversely proportional to the molecular mass. This means less will be the molecular mass, more will be the number of molecules.

CO₂ = 12 × 2 × 16 = 44u

N₂ = 2× 14 = 28u

H₂ = 2× 1 = 2u

CH₄ = 12 + 4× 1 = 16u

Hence, 1 g H₂ has maximum numbers of molecules.