Question

Assertion: Boiling point of pentane is higher than 2,2-dimethylpropane.
Reason: There is steady increase in boiling point with increase in molecular mass.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Both Assertion and Reason are incorrect

Answer 1

The temperature at which the vapour phase of a liquid equals the pressure surrounding the liquid is called the boiling point of a substance. At boiling point, the liquid changes into vapour. More the molecular mass of a compound or more is its surface area, more are the number of Vander Waal bonds to break and thus, more is their boiling point.

Complete Step by step solution:
Pentane and 2,2-dimethylpropane are isomers of each other. Pentane is a straight chain isomer and 2,2-dimethylpropane is a branched isomer. The surface area of pentane is more as compared to 2,2-dimethylpropane. This is because the length of the carbon chain is longer in pentane.
Van der Waals force is directly proportional to the surface area. So, more the surface, more will be the ability of molecules to attract each other and more energy will be required to loosen their attraction. Hence as the surface area increases, the boiling point increases. The boiling point of pentane is ${36^ \circ }$ and that of 2,2-dimethylpropane is ${9^ \circ }$. Therefore, the boiling point of pentane is higher than 2,2-dimethylpropane.
As the molecular mass increases, Van der Waals force increases. Hence the boiling point increases steadily with increase in molecular mass.
Though both assertion and reason are correct, the reason is not the correct explanation for assertion.
Hence, the correct answer is option B.

Note:
Pentane is a non-polar solvent and a refrigerant. Pentane is used as a blowing agent which is then used to create foam. The foam is known as polystyrene, polystyrene is used to make insulation materials for refrigerators and heating pipes.