Which of the following liquids, at the same temperature, has the lowest vapor pressure?
A) CH3-O-CH3
B) CH3-CH2-CH2-CH3
C) CH3-CH2-CH3
D) CH3-CH2-F
E) CH3-CH2-O-H
Solution: Vapor pressure is related to intermolecular forces. The stronger the intermolecular forces, the lower the vapor pressure is.
Intermolecular forces can be divided into 4 major categories:
1. ION-ION
This is the strongest intermolecular force. It is present between ions. Ions are easily recognized because they have a charge. Ex: Na+, Cl-, OH-...
2. HYDROGEN BONDING
Even though there is a word bonding in its name, it is still a force and not a bond. Hydrogen bonding is present in molecules that have OH, NH or FH. It is the strongest type of dipole-dipole force and is usually written separately. It occurs when a hydrogen from OH, NH or FH is attracted to the lone pair of an O, N or F of a separate molecule. Example: H2O with H2O
3. DIPOLE-DIPOLE
This type of force is present in polar molecules. Ex: H2S, CH3Cl...
A polar molecule has an asymmetrical distribution of charge. In order to recognize a polar molecule, we must draw out the Lewis dot structure first.
Mayya's Trick
A Molecule is NONPOLAR if
It consists of two of the same atoms. Example: H2, O2, N2
OR
Only carbons and hydrogens. Example: CH3CH3
OR
The central atom is attached to the same atoms AND the shape of the molecule is one of the following: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, square planar.
Example: CCl4(carbon is attached to all the same atoms, chlorines, and the shape of the molecule is tetrahedral)
4. LONDON DISPERSION
This type of force is present in all substances. However, nonpolar molecules have ONLY dispersion force while others have dispersion force and other forces.
It is also important to note that dispersion force is the weakest force. The strength of dispersion forces is connected to the molar mass. The higher the molar mass, the stronger the dispersion force will be.
For example: H2 should have weaker dispersion forces than Cl2 since it has a lower molar mass.
Choice E has an OH which means it has hydrogen bonding while the rest of the molecules have either dipole-dipole (polar molecules) or London dispersion forces(nonpolar molecules). Hydrogen bonding is strongest out of these and should have the lowest vapor pressure.
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