Multiple Choice
Which statement best helps to explain the observation that water boils at 100~^ext{o}C, whereas a 1~ext{molal} aqueous solution of NaCl boils at a higher temperature?
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14. Solutions
Boiling Point Elevation
Multiple Choice
Arrange the following compounds in order from highest boiling point to lowest boiling point: NaCl, H2O, CH3OH, CH4.
A
CH4 > CH3OH > H2O > NaCl
B
CH3OH > H2O > NaCl > CH4
C
NaCl > H2O > CH3OH > CH4
D
H2O > CH3OH > CH4 > NaCl
Verified step by step guidance1
Identify the types of intermolecular forces present in each compound: NaCl is an ionic compound with strong ionic bonds; H2O exhibits hydrogen bonding and dipole-dipole interactions; CH3OH (methanol) also has hydrogen bonding but generally weaker than water; CH4 is nonpolar and only has weak London dispersion forces.
Recall that boiling point generally increases with the strength of intermolecular forces because more energy is required to separate the molecules or ions from the liquid phase to the gas phase.
Compare the strength of forces: Ionic bonds in NaCl are the strongest, so NaCl will have the highest boiling point. Hydrogen bonding in H2O is stronger than in CH3OH due to water's ability to form more extensive hydrogen bonding networks, so H2O has a higher boiling point than CH3OH.
Since CH3OH has hydrogen bonding but less extensive than H2O, it will have a higher boiling point than CH4, which only has weak London dispersion forces.
Arrange the compounds from highest to lowest boiling point based on the above analysis: NaCl > H2O > CH3OH > CH4.
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