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Which of the following compounds has the lowest vapor pressure at room temperature?
a) H₃C-O-CH₃
b) CH₃CH₂CH₂CH₃
c) CH₃CH₂OH
d) CH₃CH₂CH₃
e) CH₃CH₂F



Answer :

amys2025

Answer:

d) CH₃CH₂CH₃

Explanation:

Vapor pressure is the pressure that is exhibited by a vapor above a liquid surface. There is a correlation between the strength of the intermolecular forces (IMFs) between molecules and the vapor pressure. The stronger the IMFs, the higher the vapor pressure.

From previous knowledge about intermolecular forces, the three main types of intermolecular forces are, in order from weakest to strongest, london dispersion forces, dipole-dipole interactions, and hydrogen bonding. All compounds have london dispersion forces, but only polar molecules have dipole-dipole interactions. Polar molecules will have hydrogen bonding if hydrogen is directly bonded to a F, N, or O.

Going down the list:

  • H₃C-O-CH₃ has both LDFs and dipole-dipole interactions, as the C-O bond is polar.
  • CH₃CH₂CH₂CH₃ is non-polar, so it only has LDFs. The C-H bond is nonpolar.
  • CH₃CH₂OH has LDFs, dipole-dipole interactions, and hydrogen bonding because the C-O bond is polar, and the -OH ending indicates an oxygen is bonded to a hydrogen in the molecule.
  • CH₃CH₂CH₃ is non-polar, so it only has LDFs.
  • CH₃CH₂F has LDFs and dipole-dipole interactions, as the C-F bond is polar. Note that it does not have hydrogen bonding, as the F in the compound is not bonded to a hydrogen, but rather a carbon in the carbon chain. If the lewis structure of the compound is drawn, this is evident.

From this, we can see that there are two non-polar compounds with just LDFs: CH₃CH₂CH₂CH₃ and CH₃CH₂CH₃. However, we can determine that one will have stronger IMFs than the other, as the number of contact points a molecule has is also a factor in determining how strong the IMFs are.

The longer the molecule, the more contact points it has. As CH₃CH₂CH₂CH₃ is longer than CH₃CH₂CH₃, CH₃CH₂CH₂CH₃ has more contact points and therefore stronger IMFs than  CH₃CH₂CH₃. This also means that CH₃CH₂CH₃ is the one that therefore has the lowest vapor pressure.

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