18g of CH₄ contains a maximum number of atoms.

Molar mass of CH₄ = The atomic mass of C + 4 X the atomic mass of H

= 12 + 4

= 16g

Number of atoms = 1 + 4 = 5

Thus,

Number of atoms = Number of atoms in the molecule×

=

= 5.63 N_A

The number of atoms in 18g of H₂O :

Molar mass of H₂O = 18g; Number of atoms = 2 + 1 = 3

=

= 3 N_A

The number of atoms in 18g of O₂:

Molar mass of O₂ = 32g; Number of atoms in O₂ = 1 + 1 = 2

=

= 1.12 N_A

The number of atoms in 18g of CO₂ :

Molar mass of CO₂ = 44g;

=

= 1.23 N_A