Question

The boiling points of ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}{\text{, C}}{{\text{H}}_{\text{3}}}{\text{OH, }}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{H}}_{\text{2}}}$ and ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{O}}_{\text{2}}}$ are ${\text{8}}{{\text{0}}^{\text{o}}}{\text{, 6}}{{\text{5}}^{\text{o}}}{\text{, 18}}{{\text{4}}^{\text{o}}}$ and ${\text{21}}{{\text{2}}^{\text{o}}}$ respectively. Which will show the highest vapour pressure at room temperature?
(A) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}$
(B) ${\text{C}}{{\text{H}}_{\text{3}}}{\text{OH}}$
(C) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{H}}_{\text{2}}}$
(D) ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{O}}_{\text{2}}}$

Answer 1

The boiling point of the liquid is inversely proportional to the vapour pressure of that liquid. Highly volatile liquids have high vapour pressure and low boiling points. Less volatile liquids have low vapour pressure and high boiling points.

Complete answer:
The chemical formulae ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}{\text{, C}}{{\text{H}}_{\text{3}}}{\text{OH, }}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{H}}_{\text{2}}}$ and ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{O}}_{\text{2}}}$ represents the compounds benzene, methanol, aniline and nitrobenzene respectively.
${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}{\text{, C}}{{\text{H}}_{\text{3}}}{\text{OH, }}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{H}}_{\text{2}}}$ and ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{N}}{{\text{O}}_{\text{2}}}$ have the boiling points of ${\text{8}}{{\text{0}}^{\text{o}}}{\text{, 6}}{{\text{5}}^{\text{o}}}{\text{, 18}}{{\text{4}}^{\text{o}}}$ and ${\text{21}}{{\text{2}}^{\text{o}}}$ respectively. Thus, methanol has the lowest boiling point and nitrobenzene has the highest boiling point.
The boiling point increases with increase in the molecular weight. Also, the compounds that can form hydrogen bonds, exhibit higher boiling points. A hydrogen bond is formed when a hydrogen atom is attached to electronegative oxygen, nitrogen or fluorine atom.
Even though methanol forms intermolecular hydrogen bonds, it shows lowest boiling point as it has lowest molecular weight.
Methanol ${\text{C}}{{\text{H}}_{\text{3}}}{\text{OH}}$ will show highest vapour pressure at room temperature as it has the lowest boiling point. Lower is the boiling point, more volatile is the compound and higher is the vapour pressure.

Hence, the option (B) ${\text{C}}{{\text{H}}_{\text{3}}}{\text{OH}}$ is the correct option.

Note: The boiling point of the liquid is the temperature at which the liquid boils. When the temperature of the liquid is equal to its atmospheric pressure, the vapour pressure of the liquid becomes equal to the atmospheric pressure. For example, the boiling point of water is ${\text{10}}{{\text{0}}^{\text{o}}}{\text{C}}$. When the temperature of water is ${\text{10}}{{\text{0}}^{\text{o}}}{\text{C}}$ it begins to boil. At ${\text{10}}{{\text{0}}^{\text{o}}}{\text{C}}$ , the vapour pressure of water is equal to the atmospheric pressure.