Explain why hydrogen fluoride HF has a higher boiling temper

Explain why hydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HC1) (19.4 degree C vs. - 85 degree C), even though HF has a lower molecular weight.

Solution

The hydrogen halides (HX ; where X=F ,Cl,Br, I), only hydrogen fluoride exhibits hydrogen bonding between molecules, and therefore has the highest melting and boiling points of the HX series. (Hydrogen bonding is the force of attraction between like flourine and the Hydrogen atom due to high electronegative nature of flourine)

Generally the boiling point rises with molecular weight (as in HCl to HI). This trend is attributed to the increasing strength of intermolecular Van der Waals forces, which correlates with numbers of electrons in the molecules.
But in HF; hydrogen bonding predominates over the van der Waals force in other hydrogen halides so HF has highest boiling point.