Question

The boiling point of ${C_6}{H_6},C{H_3}OH,{C_6}{H_5}N{H_2}{\text{ and }}{C_6}{H_5}N{O_2}$ are ${80^0}C,{65^0}C,{184^0}C,{212^0}C$ respectively.
Which will show the highest vapour pressure at room temperature?
\[
  A.{\text{ }}{C_6}{H_6} \\
  B.{\text{ }}C{H_3}OH \\
  C.{\text{ }}{C_6}{H_5}N{H_2} \\
  D.{\text{ }}{C_6}{H_5}N{O_2} \\
 \]

Answer 1

Hint: In order to solve the given problem and find out the highest vapour pressure amongst the given options, first we will understand the meaning of vapour pressure and further we will understand the characteristics over which the vapour pressure of the compound depends. On the basis of the characteristics and the boiling point of the compounds we will compare the compounds to find the correct option.

Complete step by step answer:
First let us understand the definition of vapour pressure.
Vapour pressure value is a measurement of a material's propensity to shift into a state of gaseous or vapour, and it rises with temperature. The boiling point of the liquid is considered the temperature at which the vapour pressure at the surface of a liquid becomes equal to the pressure applied by the atmosphere.
Vapour pressure of substance depends upon various physical and chemical factors but vapour pressure of organic compounds depends mainly on the boiling point of the compound.
Vapour pressure of the organic compound is inversely proportional to the boiling point of the compound.
So in order to find out the compound with highest vapour pressure we need to find the compound with lowest boiling point.
We have been given the boiling points of different compounds.
As we can see that methanol $C{H_3}OH$ has the lowest boiling point amongst the given options which is equal to ${65^0}C$ . So, methanol will have the highest vapour pressure.
Hence, the highest vapour pressure at room temperature will be shown by $C{H_3}OH$ .
So, the correct answer is “Option B”.

Note: In order to solve such types of problems students must remember the relation between the vapour pressure and boiling point that the vapour pressure is inversely proportional to the boiling point. This problem could also be solved if the boiling point was not given in the problem as it is clear that methanol was only compound in the set without a closed ring so, eventually it will have the lowest boiling point.