How do you predict the order in which these molecules will evaporate? CO2, H2O, CH3OH and C6H14

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ncchemist | eNotes Employee

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First you can separate the gasses from the liquids.  Carbon dioxide (CO2) is the only gas listed so it will evaporate the easiest.  The remaining three are liquids: water (H2O), methanol (CH3OH), and hexane (C6H14).  In general for small molecule liquids there are two forces as play here: polarity and mass (or size).  More polar compounds have higher boiling points then less polar compounds due to increased intermolecular interactions between the polar groups.  Also, the larger compounds tend to have higher boiling points than smaller compounds.  For water, it is a small compound but extremely polar so it will be the highest boiling point.  For methanol and hexane it is a bit of a toss up.  Methanol is smaller but more polar while hexane is larger but less polar.  It's hard to know which factor will dominate here but I would guess that the larger mass of hexane would make it slightly more difficult to evaporate than methanol.  In looking up the actual boiling points, the order from lowest to highest is as follows:

CO2 (-57 deg C)<CH3OH (65 deg C)<C6H14 (69 deg C)<H2O (100 deg C)


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