Question

Which of the following has the lowest boiling point?
(A) 2-Methyl-butane
(B) 2-Methyl-propane
(C) 2-2,Dimethyl-propane
(D) n-pentane

Answer 1

Hint: Boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into vapor. Boiling point increases with increase in the molecular mass.

Complete step by step answer:
Boiling point is the temperature at which the pressure exerted by the surroundings upon a liquid is equaled by the pressure exerted by the vapor of the liquid. Under this condition, addition of heat results in the transformation of the liquid into vapor without raising the temperature.
However, the types of molecules that make up a liquid determine its boiling point which means that if the intermolecular forces between molecules are relatively strong, then the boiling point will be relatively high.
Moreover, the boiling point increases with the increase in the molecular mass.
Now, let us see the molecular masses of the above mentioned compounds.
- Molecular mass of 2Methylbutane is $72gmo{l}^{-1}$
- Molecular mass of 2Methylpropane is $58gmo{l}^{-1}$
- Molecular mass of 2,2dimethylpropane is $72gmo{l}^{-1}$
- Molecular mass of n pentane is $72.1gmo{l}^{-1}$
For the compounds with the same molecular mass, boiling point decreases with an increase in branching.
Therefore, 2methylpropane has the lowest molecular mass among the following options. Hence, it has the lowest boiling point.

Hence, option B is correct.

Note: The chemical element with the lowest boiling point is helium and the element with highest boiling point is Tungsten. The vapor pressure increases with temperature because at higher temperature, the molecules move faster and have a higher tendency to overcome the attractive intermolecular forces that bind them together.