The boiling ponit of phophine is -85 oC, whilst of ammonia in the same group is -33.5 oC, how can we account for this difference in BP?

Answer 1

since PH3 has a higher molar mass, it would be expected that it would have a higher BP than NH3, but that's not the case because of hydrogen bonding in NH3 and not in PH3. it occurs when H+ is attracted to lone pairs of atoms like N, O, F in another molecules.
other molecules that exhibit H-bonding are H2O, HF, R-OH, etc.