The main reasons why ch4 has a higher vapor pressure at a given temperature when compared to ch3cl is that ch4 and . 2. the main reasons why h2co has a higher vapor pressure at a given temperature when compared to ch3oh is that h2co and . 3. the main reason why ch3oh has a higher vapor pressure at a given temperature when compared to ch3ch2ch2oh is that ch3oh .

Vapour pressure of system depends upon intermolecular forces of interaction. Greater the interaction, larger will the vapour pressure, more will be the boiling point.

Answer 1: 
CH4 stands for methane and CH3Cl named as chloromethane. In methane, all the valances of C atom is satisfied by hydrogen. Due to this, it has zero dipole moment. While, in case of CH3Cl, one of the valance is satisfied by an electronegative element i.e. Cl. Due to this, it acquires a polar character. Also, it has a net dipole moment. Due to this, CH3Cl exhibits dipole-dipole intermolecular force of attraction, which is absent in CH4.  Hence, CH3Cl has lower vapor pressure as compared to CH4. 

Answer 2: 
H2CO is named as formaldehyde, while CH3OH is named as methyl alcohol. In case of methyl alcohol, hydrogen atom (an electropositive atom) is bonded to oxygen (a highly electronegative element). This is absent in case of formaldehyde. Due the this, methyl alcohol as greater polarity as compared to formaldehyde. Due the greater polarity, vapour pressure of CH3OH is less as compared to H2CO. 

Answer 3:
CH3CH2CH2OH is named as propanol or propyl alcohol. Propyl alcohol, has longer chain length as compared to methyl alcohol (CH3OH). Both of this compounds has a polar character due to presence of -OH functional group. However, due to long chain of propyl alcohol, polar character increases. This can be attributed the +I effect of CH3 group. Due to this, intermolecular forces of interaction are higher in propanol, thereby decreasing its vapor pressure as compared to methanol.