Which has the maximum number of molecules?

1 mole of any element contains 6.023 x 10²³ number of molecules.

$$\begin{gathered} \left(1\right) 1 \mathrm{g} \mathrm{of} {\mathrm{CO}}_2- \\ \mathrm{Number} \mathrm{of} \mathrm{moles}=\frac{1}{44} \\ \mathrm{Number} \mathrm{of} \mathrm{molecules}= \frac{1}{44}\times 6.022\times {10}^{23} \\ \left(2\right) 1 \mathrm{g} \mathrm{of} {\mathrm{N}}_2 \\ \mathrm{Number} \mathrm{of} \mathrm{moles} \mathrm{of} {\mathrm{N}}_2=\frac{1}{28}\times 6.022\times {10}^{23} \\ \left(3\right) 1 \mathrm{g} {\mathrm{H}}_2 \\ \mathrm{Number} \mathrm{of} \mathrm{mole}cules \mathrm{of} {\mathrm{H}}_2=\frac{1}{2}\times 6.022\times {10}^{23} \\ \left(4\right) 1 \mathrm{g} \mathrm{of} {\mathrm{CH}}_{4 } \\ \mathrm{Number} \mathrm{of} \mathrm{mole}cules= \frac{1}{16}\times 6.022\times {10}^{23} \end{gathered}$$

   $6.022\times {10}^{23}$ is the same for all compounds, so if we compare the remaining digits in the number of moles.

$\frac{1}{44}<\frac{1}{28}<\frac{1}{16}<\frac{1}{2}$

Number of molecules of 1 g CO₂ < Number of molecules of 1 g N₂ <  Number of molecules of 1 g CH₄ < Number of molecules of 1 g  of H₂

Hence, the number of molecules is highest in the 1g H₂.