Question

How would you rank the following compounds from highest to lowest vapor pressure: $C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}OH$, $C{{H}_{3}}OH$, $C{{H}_{4}}$ and $C{{H}_{3}}C{{H}_{2}}OH$ ?

Answer 1

Hint:‌ Vapor pressure is a liquid property that is related to evaporation. The molecules have distribution of kinetic energies that are related to temperature of the system when they are present in liquid state. It is a pressure that is exerted by a vapor in equilibrium with its condensed phase.

Complete answer:
A substance that has high vapor pressure at room temperature, is considered as volatile. As we increase the temperature of a liquid, kinetic energy of a molecule also increases. The number of molecules converting into vapor also increases, which thereby increases the vapor pressure. And as we decrease the temperature of a liquid, kinetic energy of a molecule also decreases. The number of molecules converting into vapor also decreases, which thereby decreases the vapor pressure.
Now, let us discuss the answer.
$C{{H}_{4}}$ has very weak intermolecular force of attraction and therefore, it is most volatile. $C{{H}_{4}}$ has the highest vapor pressure amongst the given options.
Propanol is an alcohol, which shows hydrogen bonding and therefore, it is nonvolatile. It shows lower pressure than alkanes. As propanol has more number of carbon atoms than other given alcohols, alcohol chain lengthens and dispersion forces operate between the alkyl chain which increases the boiling point and further decreases the vapor pressure.
Ethanol has less carbon atoms that propanol, therefore it exerts more vapor pressure than propanol.
Methanol has less number of carbon atoms than ethanol, therefore it exerts more vapor pressure than ethanol.
Therefore, the rank of the given compounds from highest to lowest vapor pressure is:
\[C{{H}_{4}}>C{{H}_{3}}OH>C{{H}_{3}}C{{H}_{2}}OH>C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}OH\]

Note:In this question we concluded that if a compound shows high boiling point and less volatility, then the compound will have less vapor pressure and if a compound shows less boiling point and more volatility, then the compound will have more vapor pressure.Hydrogen bonding forms stronger bond, and therefore it produces very low evaporation rate, that eventually decreases the vapor pressure.