Microscopic world ii

It is a correct answer that HI gas a larger molecular size than HBrleads to HI has stronger van der Waals' forces between molecules.

This is to do with the intermolecular forces in the two compounds. There are no hydrogen bonds between the molecules of either compound, since Br and I are not electronegative enough to polarise the molecules sufficiently. But since HI molecules contain more electrons than HBr, there are increased van der Waals forces in HI. For the same reason HBr has a higher boiling point than HCl, but HF has a higher boiling point than HCl, HBr or HI because of hydrogen bonding.