At a certain temperature and pressure, 0.20 mol of carbon dioxide has a volume of 3.1 l. a 3.1-l sample of hydrogen at the same temperature and pressure

            A 3.1 L sample of hydrogen at the same temperature and pressure contains the same number of molecules.

Explanation:
                   As we know ;"1 mole of any gas at STP occupies exactly 22.4 L of volume". Therefore, CO₂ and H₂ gas having same number of moles (0.20 moles) will occupy exactly the same volume.

Now, converting moles into number of molecules,

# of Molecules  =  Moles × 6.022 × 10²³

Putting value of mole,

# of Molecules  =  0.2 mol × 6.022 × 10²³ molecules.mol⁻¹

# of Molecules  =  1.20 × 10²³ molecules

Hence, both gases will contain 1.20 × 10²³ molecules.