Why does CH4 have a higher vapor pressure at a given temperature than CH3Cl? Why does CH3OH have a higher vapor pressure at a given temperature than CH3CH2CH2OH? Why does H2CO have a higher vapor pressure at a given temperature than CH3OH?

Explanation:

CH4, methane is a non polar compound whereas CH3Cl is a polar compound because of presence of electronegative element chlorine. Hence, there is a net dipole moment in case of CH3Cl( Chloromethane). This causes CH3Cl to exhibits dipole-dipole intermolecular force of attraction, which is absent in CH4. Therefore, Chloromethane has a lower vapor pressure when compared to methane.  

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