Which of the following will have the highest vapor pressure?

A) Except for a substituted Hydrogen, they are basically the same molecule but as you can easily deduce, Hydrogen with an Atomic Weight of 1 vs Chlorine with a weight of 35 is all the difference and by that you can deduce Methane CH4 is lighter and would have a higher Vapor Pressure.

b) Propanol vs Methanol (CH3OH). This is pretty simple 3 Carbons 6 Hydrogens and 1 Oxygen = 58 atomic weight. Methanol has 1-Carbon and 3 Hydrogen and 1 Oxygen atom = 31 atomic weight vs. So Methanol with the lower atomic weight should have the higher vapor pressure.

c) 31 vs. 30 atomic weight. are fairly close, Formaldehyde H2CO has a Vapor pressure of 23-26mm Hg @ 25 C vs. 126.57 mm Hg for Methanol. I believe in this case is that there are stronger intramolecular forces with formaldehyde vs. methanol thus a lower vapor pressure.