How can you tell the strength of intermolecular forces based off of a chemical formula?

Here's what I get  

Explanation:

If the molecular formula contains a metal and either a nonmetal or a complex ion (as in NaCl and NaNO₃), the compound will be ionic and have strong attractive forces among its particles.

If the molecular formula contains only nonmetals (as in CO₂ or NH₃), the compound will be covalent and have relatively weak intermolecular forces.

You need more information than just the molecular formula to make predictions about other types of intermolecular forces such as dipole-dipole and London dispersion forces.