The following best helps to explain that the pressure of a sample of CH₄ is closer to the pressure predicted by gas law than a sample of NH₃ is NH₃ molecules are polar while CH₄ molecules are not.
The methane mole , CH₄  is the non polar molecule.  the bond present between the  CH₄  is the covalent bond. the compound is called as the covalent compound. the covalent compound is formed by the sharing of the electron pairs. The  NH₃ is polar molecule. the  NH₃ is also form the covalent bond but it has net dipole moment so,  NH₃ is the polar covalent bond.
Thus, the pressure of a sample of CH₄ (molar mass 16g/mol) is closer to the pressure  by the ideal gas law than a sample of NH₃  (molar mass 17g/mol):  NH₃  molecules are polar while CH₄ molecules are not.
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