(a)Give the decreasing order of melting points of the following `NH_(3),PH_(3),(CH_(3))_(3)N` Explain
(b) In which molecule is the van der Waals force likely to be the most important in determining the m.pt and b.pt for `ICI,Br_(2),HCI,H_(2)S,CO` .

Explanation

`NH_(3)` (due to H-bonding) gt `(CH_(3))_(3) N gt PH_(3)`
Melting points `= - 77.7^(@)C gt - 117^(@)C gt - 133^(@)C`
`Br_(2)`Each of other molecules ha a dipole in addition to van der Waals forces Only in `Br_(2)` van der Waals are the only intermolecular forces .