Question

The boiling points of methanol, water and dimethyl ether are $338K$ , $373K$ and $307.5K$ respectively. Which one of the following statements gives the best explanation for these wide variations in b.p.?
A. The molecular mass increases from water $\left( {18} \right)$ to methanol $\left( {32} \right)$ to dimethyl ether $\left( {74} \right)$ .
B. The extent of intermolecular $H$ -bonding decreases from water to methanol and it is absent in ether.
C. The extent of intramolecular $H$ -bonding decreases from ether to methanol to water.
D. The density of water is greater than that of methanol which is in turn less than dimethyl ether.

Answer 1

The changing boiling point of the different chemical molecules take place due to changing physical properties of the liquids. These physical properties include the bond formation and interaction of the molecules, which affects the various changes that take place during the shifting of states of matter.

Complete solution
One of the most important characters which differ in the different given chemicals due to which the boiling point changes are hydrogen bonding between the specific molecules. Higher the chances of formation of a hydrogen bond, higher will be the boiling point of the specific chemical. There are chances of formation of hydrogen-bonded structures between the molecules of methanol and water. These two solvents can properly form the bonded structure and due to the $H -$ bonded form and hence this interaction does not let the solvent to be converted to gaseous form easily. The interaction holds the chemicals in the form of liquids and only at a very high temperature the change of state occurs like that of methanol and water. This condition is not observed in the dimethyl ether as there is no chance of formation of hydrogen bond and that is why it has a lower boiling point of $307.5K$ .
The number of intermolecular hydrogen bonds differs between methanol and water. The intermolecular hydrogen bond is higher in water as compared to methanol and that is why there is a higher level of interaction between the molecules. There is an added inductive effect $\left( { + I} \right)$ which results in higher density of negative charge on the oxygen residue. This is why the water has a higher boiling point as compared to methanol. Therefore, the correct option is B. The extent of intermolecular $H$ -bonding decreases from water to methanol and it is absent in ether.

Hence, option B is correct.

Note
The pattern of interaction differs in the molecules even if the mode of interaction between the molecule remains similar. This can differentiate between the molecules properly based on the physical nature of the component and also the condition in which it exists in nature.