Question

The boiling point of ${C_6}{H_6},C{H_3}OH,{C_6}{H_5}N{H_2}{\text{ and }}{C_6}{H_5}N{O_2}{\text{ }}$ are $80^\circ C,65^\circ C,184^\circ C{\text{ and }}212^\circ C$ respectively. Which will show the highest vapour pressure at room temperature.
A.${C_6}{H_6}$
B.$C{H_3}OH$
C.${C_6}{H_5}N{H_2}$
D.${C_6}{H_5}N{O_2}$

Answer 1

The boiling point and vapour pressure have a unique relation in them. The temperature at which the vapour pressure of the liquid is equal to the vapour pressure of the surface of the liquid is known as its boiling point.

Complete step by step answer:
The temperature at which the vapours of liquid have pressure equal to the pressure of the surrounding is known as its boiling point. At this temperature the liquid starts converting into vapours.
The tendency of a matter to change into vapour form or vapour state is known as its vapour pressure. Basically it is the pressure exerted by the particles of matter in vapour form. When this vapour pressure becomes equal to the atmospheric pressure or pressure of the surrounding environment then it is termed as its boiling point. Vapour pressure depends on the temperature, the vapour pressure gradually increases along with the temperature.
As we understand that vapour pressure and boiling point have a unique relation in them, now we will try to understand how it works.
When the vapour pressure becomes equal to the pressure present on the surface of liquid it is known as boiling point, so we can say that higher the vapour pressure lower the boiling point.. Hence compounds with lower boiling point have high vapour pressure and vice versa.
As we have provided in the problem that methanol has the lowest boiling point i.e. $65^\circ C$ so methanol will show the highest vapour pressure at room temperature.
So the correct answer is option B.

Note:
The boiling point of organic compounds is affected by three factors mostly. First is the intermolecular force, second is the number of carbon atoms present in the compound and third is branching, branching decreases the boiling point. ${\text{No}}{\text{. of carbon atoms}} \propto {\text{ Boiling point}}$.