Question

Melting and boiling point of alkanes increases with increase in molar mass. why?

Answer 1

The longer the alkane gets (higher molecular weight), the more intermolecular forces are present (London Dispersion), and this increases the melting and boiling points.

Explanation:

Increasing molar mass usually increases boiling point. The noble gases are examples of this. Basically, molecules/atoms need to acquire enough kinetic energy (temperature) to escape into the gas phase (boiling), and if that molecule/atom is heavy, it can't "get wings" and get into the gas phase. So that is molecular mass.

The longer a carbon chain gets in an alkane, the more small london dispersion forces it will have. These interactions increase the melting and boiling points.