Question

Which one of the following has the lowest boiling point?
A. $C{H}_{3}Cl$
B. $C_2{H}_{5}Cl$
C. $C_2{H}_{5}Br$
D. $C_2{H}_{5}I$

Answer 1

Hint: Boiling point of a substance or liquid is defined as the temperature at the vapour pressure of the substance in liquid phase is equal to the pressure of the surrounding above the liquid and the liquid changes to the vapour.

Complete step by step answer: Boiling point depends on various factors. It depends on ionic bonds, intermolecular forces, shape of molecules and size of molecules. The factors are influenced by the type of functional group attached to the molecule.
Further boiling point is characteristic of the strength of the bond present between the molecules. If the bond is strong more will be energy required to break the bong and higher will be the boiling point.
For the given set of molecules, all of them are alkyl halides. The key point to the boiling of these molecules lies in the difference of their molecular weights. In general the higher the molecular weight or molar mass the more is the boiling point. Alternatively, the lower the molar mass, the lower is the boiling point.
Boiling point also increases with the number of carbon atoms in a straight chain. Branching however leads to decrease of boiling point due to decrease in surface area.
The molar mass of the given molecules increases in the order $$C{H}_{3}Cl<C_2{H}_{5}Cl<C_2{H}_{5}Br<C_2{H}_{5}I$$, i.e. $$50.49g<64.51g<108.97g<155.97g$$. As boiling point increases with increase in molar mass, so the boiling point order is $$C{H}_{3}Cl<C_2{H}_{5}Cl<C_2{H}_{5}Br<C_2{H}_{5}I$$.

Thus, option A is the correct answer, i.e. $C{H}_{3}Cl$ has the lowest boiling point.

Note:Like boiling point melting point of a solid also varies depending on the same factors. Under intermolecular forces, the forces caused by hydrogen bonding are the strongest and lead to the higher boiling point among the molecules.