5. If the molecules have similar molar masses and similar types of intermolecular forces, look for the one that is the most polar or that has the most electronegative atoms or the most hydrogen bonding groups. That one will have the strongest IMF's overall.
Keeping this in mind which molecules have the strongest intermolecular forces?
The strongest intermolecular force is hydrogen bonding, which is a particular subset of dipole-dipole interactions that occur when a hydrogen is in close proximity (bound to) a highly electronegative element (namely oxygen, nitrogen, or fluorine).
With this in view how can you tell if a substance has strong or weak intermolecular forces?
Substances that have strong intermolecular forces are very attracted to one another and are tightly held together. These substances take a lot of energy to separate, while substances with weak intermolecular forces are very loosely held together and have weak interactions with one another.