Which has the highest molar heat of vaporisation?
A. $HF$
B. $HCl$
C. $HBr$
D. $HI$

Hint: The energy required to convert a liquid to gas is known as molar heat of vaporisation. In the presence of hydrogen bonding molar heat of vaporisation increases.

Complete Step by Step Answer:
The halogen hydride which can form hydrogen bonds has the highest molar heat of vaporisation.
The Conditions for making hydrogen bonds are as follows:
- The molecule must contain a highly electronegative atom that is attached to the hydrogen atom to form a covalent bond.
- Higher the electronegativity more is the polarisation of the molecule so that electrons of the covalent bonds are shifted towards the more electronegative atom and make partially positively charged hydrogen atoms.
- Among the halogen hydrides only hydrogen fluoride contains hydrogen bonding between electronegative fluorine and electropositive hydrogen atom. It has different properties from other halogen hydrides. That’s why hydrogen fluoride has the strongest bond and it has the highest heat of vaporisation.
- Thus we can write that $HF$ has the highest molar heat of vaporisation.
Thus the correct option is A.

Note: Hydrogen bonds can be defined as the attractive force which binds the hydrogen atom of one molecule with the electronegative atom fluorine, oxygen and nitrogen of another molecule. Hydrogen bond is the strongest bond and higher energy is required to break it.